What's the purpose of using & before a function's argument?

Question

I saw some function declarations like this:

function boo(&$var){  ... } 

what does the & character do?

Answer 1

It's a pass by reference. The variable inside the function "points" to the same data as the variable from the calling context.

function foo(&$bar) {   $bar = 1; }  $x = 0; foo($x); echo $x; // 1 

Answer 2

Basically if you change $var inside the function, it gets changed outside. For example:

$var = 2;  function f1(&$param) {     $param = 5; }  echo $var; //outputs 2 f1($var); echo $var; //outputs 5