Define whether a variable is passed by value or reference

 
<?php
function f1($a) {
  $a += 4;
}
function f2(&$a) {
  $a += 10;
}
$b = 5;
f1(&$b);
f2($b);
echo "\$b = $b\n";
?>

1.	Functions and References
2.	A function that takes a reference for an argument
3.	Declaring a Function with Arguments
4.	Function with default parameters
5.	Passing Variables as a Reference to the Argument of a Function
6.	Passing an Argument to a Function by Value
7.	Using a Function Call to Pass an Argument to a Function by Reference
8.	Declaring a Function That Requires Arguments
9.	Function with an Optional Argument
10.	A Function with an Optional Argument
11.	Accessing Function Parameters
12.	All optional arguments
13.	Capturing a return value
14.	Changing argument values
15.	Defining and Calling a two-argument function,
16.	Function Arguments
17.	Function with two parameters
18.	Creating Functions That Take a Variable Number of Arguments
19.	Creating a capitalize function with a default parameter $each
20.	Default Parameters
21.	Passing By Reference
22.	Making arguments be passed by reference
23.	Using Optional Parameters

Define whether a variable is passed by value or reference : Parameters « Functions « PHP