• The increment operators are used to increment a variable's value. • The decrement operators are used to decrement a variable's value.

$x++; // equivalent to $x = $x+1;
$x--; // equivalent to $x = $x-1; 

• Increment and decrement operators either precede or follow a variable.

$x++; // post-increment 
$x--; // post-decrement 
++$x; // pre-increment 
--$x; // pre-decrement

• The difference is that the post-increment returns the original value before it changes the variable, while the pre-increment changes the variable first and then returns the value. For instance:

$a  = 2; $b = $a++; // $a=3,  $b=2
$a  = 2; $b = ++$a; // $a=3,  $b=3

$a  = 2; $b = $a--; // $a=1,  $b=2
$a  = 2; $b = --$a; // $a=1,  $b=1