PHP supports the basic data types of strings, integers, double precision floating
point numbers, etc, but is a weakly typed language. This means that variables
are not tied to a specific data type, and may take any of the data types. Variables
are not declared in PHP, but must be prefixed with a $
sign.
String concatenation is achieved in PHP using the dot (.) operator. The following example uses a variable called anyDataType. The data type is given a string literal, and an integer values. The output is displayed using the dot operator to concatenate a break to each line printed.
<!DOCTYPE HTML PUBLIC "//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Using Variables</title>
<meta httpequiv="ContentType" content="text/html;
charset=ISO88591">
</head>
<body>
<p>
<?php
// Use several data types with one variable
$anyDataType = "Hello world";
print $anyDataType . "<br>";
$anyDataType = 64;
print $anyDataType . "<br>";
$anyDataType += 1900;
print $anyDataType . "<br>";
?>
</p>
</body>
</html>
Operators
Arithmetic Operators
Operator  Description 
+

Unary plus, or addition. 


Unary minus or subtraction. 
*

Multiplication. 
/

Division. 
%

Modulus (remainder from a divide operation). 
++

Increment (preincrement if placed before the variable, postincrement if placed after). 


Decrement (predecrement if placed before the variable, postdecrement if placed after). 
Arithmetic Operator Examples
// Caluclate the sum of a and b
$total = $a + $b;
// Calculate the difference between a and b
$difference = $a  $b;
// Calculate the product of a and b
$product = $a * $b;
// Calculate the quotient of a and b
$quotient = $a / $b;
// Calculate the remainder when a is divided by b
$remainder = $a % $b;
// Increment the value of a
$a++;
// Decrement the value of b
$b;
// The preinrecrement version
$num = ++$a;
// The predecrement version
$num = $b;
Self Assignment
If an assignment is made that requires the value of the variable being assigned to, PHP offers a convenient shortcut by placing the operator immediately before the assignment operator.
$total += $a; // Equivalent to $total = $total + $a;
$difference = $a; // Equivalent to $difference = $difference  $a;
$product *= $a; // Equivalent to $product = $product * $a;
$quotient /= $a; // Equivalent to $quotient = $quotient / $a;
$remainder %= $a; // Equivalent to $remainder = $remainder % $a;
Relational Operators
Relational operators (sometimes called comparison operators) are used to compare values in an expression. The return value will be either true or false.
PHP Relational Operators
Operator  Description 
<

Less than (eg. $x < 10 ). 
<=

Less than or equal (eg. $x <= 10 ). 
==

Equivalence (eg. $x == 10 ). 
>

Greater than (eg. $x > 10 ). 
>=

Greater than or equal (eg. $x >= 10 ). 
!=

Not equivalent (eg. $x != 10 ). 
Logical Operators
Logical operators allow you to combine the results of multiple expressions to return a single value that evaluates to either true or false.
PHP Logical Operators
Operator  Description 
!

Logical Not. For example, (!$x) will evaluate to True if $x is zero. 
&&

Logical And. For example, ($x > 4 && $y <= 10) will evaluate
to True is $x is greater than four and $y is less than or equal to ten. 


Logical Or. For example, ($x > 10  $y > 10) will evaluate to
True is either $x or $y are greater than ten. 
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