The basic assignment operator is "=". Your first inclination might be to think of this as "equal to". Don't. It really means that the left operand gets set to the value of the expression on the right (that is, "gets set to").
The value of an assignment expression is the value assigned. That is, the value of "$a = 3" is 3. This allows you to do some tricky things:
<?php
$a = ($b = 4) + 5; // $a is equal to 9 now, and $b has been set to 4.
?>
In addition to the basic assignment operator, there are "combined operators" for all of the binary arithmetic, array union and string operators that allow you to use a value in an expression and then set its value to the result of that expression. For example:
<?php
$a = 3;
$a += 5; // sets $a to 8, as if we had said: $a = $a + 5;
$b = "Hello ";
$b .= "There!"; // sets $b to "Hello There!", just like $b = $b . "There!";
?>
Note that the assignment copies the original variable to the new one (assignment by value), so changes to one will not affect the other. This may also have relevance if you need to copy something like a large array inside a tight loop.
An exception to the usual assignment by value behaviour within PHP occurs with objects, which are assigned by reference in PHP 5. Objects may be explicitly copied via the clone keyword.
Assignment by reference is also supported, using the "$var = &$othervar;" syntax. Assignment by reference means that both variables end up pointing at the same data, and nothing is copied anywhere.
Example #1 Assigning by reference
<?php
$a = 3;
$b = &$a; // $b is a reference to $a
print "$a\n"; // prints 3
print "$b\n"; // prints 3
$a = 4; // change $a
print "$a\n"; // prints 4
print "$b\n"; // prints 4 as well, since $b is a reference to $a, which has
// been changed
?>
As of PHP 5, the new
operator returns a reference automatically, so assigning the result of
new by reference results
in an E_DEPRECATED message in PHP 5.3 and later, and
an E_STRICT message in earlier versions.
For example, this code will result in a warning:
<?php
class C {}
/* The following line generates the following error message:
* Deprecated: Assigning the return value of new by reference is deprecated in...
*/
$o = &new C;
?>
More information on references and their potential uses can be found in the References Explained section of the manual.