Explain function call by reference

In case of call by reference, actual value is modified if it is modified inside the function. In such case, we need to use & symbol with formal arguments. The & represents reference of the variable.

Example:

function adder(&$str2) {  
    $str2 .= 'Call By Reference';  
}
$str = 'This is ';  
adder($str);  
echo $str;  

Output:

This is Call By Reference