Transcript b = $a

PHP Basics - 2
PHP Functions: Testing Variables
As the appearance of the variable does not show what variable it
contains , many times in your scripts you won't be sure if the variable
contains a value, or even if it exists at all .
isset( )
This function tests whether a variable has any value, including an empty string. It returns
a value of either true or false.
empty( )
It is same as isset( ) function. It tests true if a variable is not set, contains an empty string
or has a value of 0.
is_int( )
This tests whether a variable is an integer.
Example:
<?php
$a = 10;
echo isset($a);
echo "<br/>";
echo is_int($a);
?>
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Testing Variables
is_double( )
It tests whether a variable is floating-point number.
is_string( )
It tests whether a variable is a text string.
is_array( )
It tests whether a variable is an array.
is_bool( )
It tests whether a variable is boolean.
is_object( )
Returns true if the variable is an object.
gettype( )
It will tell you the type of variable you have.
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Changing Variable Types
 PHP does not require explicit type definition in
variable declaration.
 A variable's type is determined by the context in
which that variable is used.


If you assign a string value to variable $my_var, $my_var
becomes a string.
If you then assign an integer value to $my_var, it becomes an
integer.
 There are ways to change the type of any variable:



Using settype( )
Type casting
Using the functions intval( ), doubleval( ) and stringval( )
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Changing Variable Types: settype
Example:
<?php
$my_var = 1995;
echo $my_var . "<br/>";
echo "The variable \$my_var is now a " . gettype($my_var) . "<br/>";
settype($my_var,"string"); echo $my_var . "<br/>";
echo "The variable \$my_var is now a " . gettype($my_var) . "<br/>";
?>
Output:
1995
The variable $my_var is now a integer
1995
The variable $my_var is now a string
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Changing Variable Types: settype
Possibles values for the settype() function are:
"boolean" (or, since PHP 4.2.0, "bool")
"integer" (or, since PHP 4.2.0, "int")
"float" (only possible since PHP 4.2.0, for older versions use the
deprecated variant "double")
"string"
"array"
"object"
"null" (since PHP 4.2.0)
The settype() function returns TRUE on success or FALSE on
failure. This is important, for example, if you want to
conditionally execute some code based on the success or
failure of the settype() function.
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Changing Variable Types: Type Casting
Type casting in PHP works much as it does in C: the name of the
desired type is written in parentheses before the variable which is to be
cast. For example:
<?php
$a =
echo
$b =
echo
?>
1;
"\$a is a " . gettype($a) . "<br />\n";
(string) $a;
"\$b is a " . gettype($b) . "<br />\n";
The approach above forces the variable value to change to the type
indicated prior to assignment and only for the purposes of the
assignment operation.
A cast does not change the variable’s type permanently.
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Type Casting (ctd)
The casts allowed in PHP are:
(int), (integer) - cast to integer
(bool), (boolean) - cast to boolean
(float), (double), (real) - cast to float
(string) - cast to string
(array) - cast to array
(object) - cast to object
Some alternative casts are shown above.
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PHP Variable Handling
Here are some examples that illustrate PHP's flexibility with respect
to variable type:
<?php
$my_var1 = 20;
echo $my_var1 . "<br>";
echo "The variable \$my_var1 is a " . gettype($my_var1) . "<br>";
$my_var2 = "5";
echo $my_var2 . "<br>";
echo "The variable \$my_var2 is a " . gettype($my_var2) . "<br>";
$my_var3 = $my_var1 * $my_var2;
echo $my_var3 . "<br>";
echo "The variable \$my_var3 is a " . gettype($my_var3) . "<br>";
$my_var4 = $my_var1 . $my_var2;
echo $my_var4 . "<br>";
echo "The variable \$my_var4 is a " . gettype($my_var4) . "<br>"; ?>
Basically, PHP assumes that you know what you want to do and
changes the variable type to allow you to do it.
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PHP Variable Variables!
Sometimes it is convenient to be able to have variable variable names.
That is, a variable name which can be set and used dynamically. A
normal variable is set with a statement such as:
$a = "hello";
A variable variable takes the value of a variable and treats that as the
name of a variable. In the above example, hello, can be used as the
name of a variable by using two dollar signs. i.e.
$$a = "world";
 Creates a variable called $hello=“world”
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PHP Variable Variables (ctd)
At this point two variables have been defined and stored in the PHP
symbol tree: $a with contents "hello" and $hello with contents "world".
Therefore, this statement:
echo "$a ${$a}";
produces the exact same output as:
echo "$a $hello";
i.e. they both produce: hello world.
•In order to use variable variables with arrays, you have to resolve an
ambiguity problem. That is, if you write $$a[1] then the parser needs
to know if you meant to use $a[1] as a variable, or if you wanted $$a
as the variable and then the [1] index from that variable.
•The syntax for resolving this ambiguity is: ${$a[1]} for the first case
and ${$a}[1] for the second.
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PHP Operators
 There are many operators used in PHP, just as
is the case in other programming languages.
 To make it easier to learn them all, we have
separated the PHP operators into categories
as follows:
 Arithmetic
Operators
 Assignment Operators
 Comparison Operators
 Logical Operators
 String Operators
 Bitwise Operators
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Arithmetic Operators
Artihmetic operators generally mirror those available in C.
Operator
Description
Example
Result
$a + $b
Addition
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$a - $b
Subtraction
$a * $b
Multiplication
$a / $b
Division
$a % $b
Modulus
(division remainder)
$a++
Increment
$a--
Decrement
$a=2
$a+2
$a=2
5-$a
$a=4
$a*5
15/5
5/2
5%2
10%8
10%2
$a=5
$a++
$a=5
$a--
3
20
3
2.5
1
2
0
$a=6
$a=4
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Arithmetic Operators: Example
The PHP code segment below illustrates some of the basic arithmetic operations.
<?php $addition = 2 + 4;
$subtraction = 6 - 2;
$multiplication = 5 * 3;
$division = 15 / 3;
$modulus = 5 % 2;
echo "addition: 2 + 4 = " . $addition . "<br />";
echo "subtraction: 6 - 2 = " . $subtraction . "<br />";
echo "multiplication: 5 * 3 = " . $multiplication . "<br />“;
echo "division: 15 / 3 = " . $division . "<br />";
echo "modulus: 5 % 2 = " . $modulus . "<br />";
?>
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Arithmetic Operators: Example
You can also specify whether you want the increment to occur while the line of
code is being executed or after the line has executed.
The PHP code segment below illustrates the difference.
<?php
$x =
echo
echo
echo
echo
?>
4;
"The
"<br
"<br
"<br
value of x with
/> The value of
/> The value of
/> The value of
post-incrementing = " . $x++;
x after the post-increment is " . $x; $x = 4;
x with with pre-incrementing = " . ++$x;
x after the pre-increment is " . $x;
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Assignment Operators
The basic assignment operator is the equal symbol ( = ). This operator
can be "read" as "the left operand is assigned the value of the expression
on the right" or "the left operand gets set to the value of the expression
on the right."
The table below lists all of the PHP operators that combine assignment
with arithmetic.
Operator
Example
Is The Same As
=
$x = $y;
$x = $y;
+=
$x += $y;
$x = $x + $y;
-=
$x -= $y;
$x = $x - $y;
*=
$x *= $y;
$x = $x * $y;
/=
$x /= $y;
$x = $x / $y;
%=
$x %= $y;
$x = $x % $y;
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Assignment Operators: Example
Example:
<?php
$counter = 8;
$counter = $counter + 1;
echo $counter;
?>
However, there is a shorthand version for incrementing that uses one
of the assignment operators above. We could write:
<?php
$counter = 8;
$counter += 1;
echo $counter;
?>
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Assignment Operators: Example
To add one to a variable or "increment" we could use the ++ operator. Thus,
we have three possible approaches to incrementing:
<?php
$x = 15;
$x++;
echo $x;
$x += 1;
echo $x;
$x = $x + 1;
echo $x;
?>
PHP's assignment operators enable you to create more concise code. The
following example illustrates this feature.
<?php
$my_var
$my_var
$my_var
$my_var
$my_var
$my_var
?>
= 12;
+= 14; // $my_var now equals 26
-= 12; // $my_var now equals 14
*= 10; // $my_var now equals 140
/= 7; // $my_var now equals 20
%= 6; // $my_var now equals 2
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Assignment Operators: Example
An assignment expression is replaced by its value. Thus, the value of the
assignment expression $a = 3 is 3. This allows you to do some interesting
things.
Example:
<?php
/*
note that $a and $b are used without declaration or
initialization
*/
$a = ($b = 3) + 5; // $b has been set to 3, $a has been set to 8
echo "\$a = $a <br/>\n";
echo "\$b = $b <br/>\n";
?>
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Comparison Operators
Comparison operators are used to compare expressions (e.g., values,
variables) in a Boolean manner. This means that an expression
constructed with comparison operators will return either true or false.
Operator
Description
Example
$a == $b
is equal to
5==8 returns false
$a === $b
is identical to
8===8 returns true
$a != $b
is not equal
5!=8 returns true
$a <> $b
is not equal
5!=8 returns true
$a !== $b
is not identical
5!=="5" returns true
$a > $b
is greater than
5>8 returns false
$a < $b
is less than
5<8 returns true
$a >= $b
is greater than or equal to
5>=8 returns false
$a <= $b
is less than or equal to
5<=8 returns true
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Comparison Operators (ctd)
Comparison Operator Values:
True will display as the integer one ( 1 ) and false will display as the empty
string.
The following values are evaluated as false:
the integer zero ( 0 )
the float zero ( 0.0 )
the empty string ( "" )
the string zero ( "0" )
an array with zero elements
an object with zero member variables
NULL
All other values are evaluated as true.
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Logical Operators
Logical operators allow you to combine the results of multiple comparison
expressions to return a single value that evaluates to either true or false.
Operator
Name
Example
$a && $b
AND
TRUE if both $a and $b are TRUE
$a=6; $b=3;
($a < 10 && $b > 1) returns true
$a and $b
AND
TRUE if both $a and $b are TRUE
$a=6; $b=3;
($a < 10 and $b > 1) returns true
$a || $b
OR
TRUE if either $a or $b is TRUE
$a=6; $b=3;
($a==5 || $b==3) returns true
($a==5 || $b==5) returns false
$a or $b
OR
TRUE if either $a or $b is TRUE
$a=6; $b=3;
($a==5 or $b==5) returns false
$a xor $b
XOR
TRUE if either $a or $b is TRUE, but not both
$a=6; $b=3;
($a>5 xor $b<5) returns false
!$a
NOT
TRUE if $a is FALSE
$a=6; $b=3;
!($a==$b) returns true
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String Operators
PHP has only two string operators (actually, only one with the second
being a derivative of the first).
The string operators are shown in the table below.
Operator
Description
Example
$a . $b
concatenation
$a = "Hello ";
$b = $a . "World!";
assigns "Hello World" to
$b
$a .= $b
concatenation and assignment
$a = "Hello ";
$a .= "World!"; assigns
"Hello World" to $a
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Bitwise Operators
Bitwise operators are used in connection with logical operations and
binary math. These operators directly affect the binary or ASCII
representations of stored data.
Operator
Description
Result
$a & $b
AND
Bits that are set in both $a and $b are set
$a | $b
OR
Bits that are set in either $a or $b are set
$a ^ $b
XOR
Bits that are set in $a or $b but not both are set
~ $a
NOT
Bits that are set in $a are not set, and vice versa
$a << $b
shift left
Shift the bits of $a $b steps to the left (each step
means "multiply by two")
$a >> $b
shift right
Shift the bits of $a $b steps to the right (each
step means "divide by two")
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Operator Precedence
The order of precedence establishes a "priority" for operators, where those with the
highest priority will be evaluated first.
When there are several operators of the same priority, they will be processed from left
to right if they are left associative, and right to left if they are right associative.
Associativity
non-associative
non-associative
right
left
left
left
non-associative
non-associative
left
left
left
left
left
right
left
left
left
Operators
Additional Information
++ -~ - (int) (float) (string) (array)
(object) (bool)
increment/decrement
!
logical
arithmetic
arithmetic and string
bitwise
comparison
comparison
bitwise and references
bitwise
bitwise
logical
logical
assignment
logical
logical
logical
* / %
+ - .
<< >>
< <= > >= <>
== != === !==
&
^
|
&&
||
= += -= *= /= .= %= &= |= ^= <<= >>=
and
xor
or
types
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Operator Precedence: Example
You can override the order of precedence but enclosing part of an expression
in parentheses. The innermost parentheses are processed first, so that:
(7 - 4) + 1 will give a result of 4
7 - (4 + 1) will give a result of 2
2 + 3 * 3 will give a result of 11
2 + (3 * 3) will also give 11
(2 + 3) * 3 will give a result of 15
1 + 3 + 3 * 2 will give a result of 10
1 + ((3 + 3) * 2) will give a result of 13
(1 + (3 + 3)) * 2 will give a result of 14
It is recommended that you use parentheses whenever you create a complex
expression (even if not needed) so that it is easier for you and others to see
what is going on when you read the code itself.
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Control Structures
 Control Structures are at the core of programming logic.
 They allow a PHP script to alter the normal top-to-bottom order of
execution for program code. In other words, the script can react
differently depending on what has already occurred, or based on user
input, and allow the graceful handling of repetitive tasks.
 The two basic control structures, present in any procedural language,
are branching and looping.

Branching allows the program to select between two or more alternative
blocks of code.


The if and switch ... case statements are used for branching.
Looping (also known as iteration) allows the program to repeat a block of
code, either a fixed number of times or until a conditional expression
evaluates to a desired value.

The for and while statements are used for looping.
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If Statement
The if statement is the most fundamental control structure in any
programming language. Its function is to execute a statement or block of
statements if and only if the comparison expression provided to it evaluates to
true.
if (conditions)
{
// Code if condition is true
}
That is, the if statement "guards" a statement that is either executed or not ...
based on the value of the expression
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If Then Else Statement
A variation on the basic if is the if ... then ... else statement. This
variation selects between two alternative statements as illustrated
below.
if (conditions)
{
// Code if condition is true
}
else
{
// Code if condition is false
}
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Examples
We can use comparison operators and logical operators in if
statements that are more complex then which we have seen at the
beginning of this chapter.
Example -1 :
if ($num1 == 1 && $num2 <=5 && !empty($num3)
{
// enter your code here
}
Example-2:
if ($a > $b)
{
print "a is bigger than b";
}
else
{
print "a is NOT bigger than b";
}
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If … ElseIf Statement
Like else, the elseif statement extends an if statement to execute a
different statement in case the original if expression evaluates to
false.
However, unlike else, it will execute that alternative expression
only if the elseif conditional expression evaluates to true.
if ($a
{
print
}
elseif
{
print
}
else
{
print
}
> $b)
"a is bigger than b";
($a == $b)
"a is equal to b";
"a is smaller than b";
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Alternative If Statements
You can write if statements in a couple of different ways. The first simply
substitutes a colon for the opening curly brace and the word endif with a
semicolon for the closing curly brace. Example:
if (condition1) :
statement1;
elseif (condition2):
statement2;
else:
statement3
endif;
The other alternative if structure is Ternary Operator, represented by question
mark character ( ? ). This character can be used as shorthand for simple
if/else Statements. It can only be used in situations where you want to
execute a single expression based on whether a single condition evaluates to
true or false. Example:
(condition) ? (executes if the condition is true) :
(executes if the condition is false);
Note that this is a single statement, with the semicolon at the end.
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Switch Statement
The switch statement is similar to a series of IF statements on the
same expression. In many occasions, you may want to compare the
same variable (or expression) with many different values, and execute
a different piece of code depending on which value it equals to. This is
exactly what the switch statement is for.
switch ($i)
{
case 0:
print "i equals 0";
break;
case 1:
print "i equals 1";
break;
default:
print “Unexpected value for \$i";
}
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While Loop
While tells PHP to execute the nested statement (s) repeatedly, as long as the
while expression evaluates to true. The value of the expression is checked
each time at the beginning of the loop. Example:
while (condition)
{
//code to execute
}
Example:
$a=0;
While ($a<=10)
{
echo “$a <br> \n”;
$a++;
}
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Do … While Loop
It is same as while loop discussed above. The only difference is that
the condition is tested after the code in question has been executed
once .
do
{
// code to be used here
} while (condition) ;
Example:
$a=0;
do
{
echo “$a <br> \n”;
$a++;
} while ($a<=10);
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For Loop
The for loop takes three expressions. The first one is initialization condition,
second is condition that is evaluated in each iteration; if this condition is false
than the loop will end, and the third expression is executed in every iteration.
Example:
for ($i = 0 ; $i <= 10 ; $i++)
{
echo $i. "<br>\n" ;
}
The first time, value of i = 0, so 0 will be printed and further 1 to 10 will be
printed as value of i is incremented by 1 in each iteration.
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Foreach Loop
PHP 4 (not PHP 3) includes a foreach construct, much like Perl and
some other languages. This simply gives an easy way to iterate over
arrays. There are two syntaxes; the second is a minor but useful
extension of the first:
foreach(array_expression as $value) statement
foreach(array_expression as $key => $value) statement
The first form loops over the array given by array_expression. On each
loop, the value of the current element is assigned to $value and the
internal array pointer is advanced by one (so on the next loop, you'll be
looking at the next element).
The second form does the same thing, except that the current
element's key will be assigned to the variable $key on each loop (for
associative arrays).
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Foreach Loop: Example
$names= array(“Jay”, “Brad”, “Ernie”, “John”);
foreach($names as $i)
{
echo $i . “<br> \n”;
}
Output:
Jay
Brad
Ernie
John
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“continue” in Loops
• Continue is used within looping structures to skip the rest of the current loop
iteration and continue execution at the beginning of the next iteration.
• Continue accepts an optional numeric argument which tells it how many
levels of enclosing loops it should skip to the end of.
Example :
$i = 0;
while ($i++ < 5)
{
echo "Outer-”.$i.”<br>\n";
while (1)
{
echo "Middle<br>\n";
while (1)
{ echo "Inner<br>\n";
continue 3;
}
echo "This never gets output.<br>\n";
}
echo "Neither does this.<br>\n";
}
Output:
Outer-1
Middle
Inner
Outer-2
Middle
Inner
Outer-3
Middle
Inner
Outer-4
Middle
Inner
Outer-5
Middle
Inner
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“break” in Loops
break ends execution of the current for , foreach , while , do..while or switch
structure. break accepts an optional numeric argument which tells it how
many nested enclosing structures are to be broken out of.
Example:
$i = 0;
while (++$i)
{
switch ($i)
{ case 5:
echo "At
break 1;
case 10:
echo "At
break 2;
default:
break;
}
}
5<br>\n";
/* Exit only the switch. */
10 quitting<br>\n";
/* Exit the switch and the while. */
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