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

Concept of the call by reference with the help of examples.

PHP Call By Reference Example 1

Variable $str is passed to the adder function where it is concatenated with the 'Call By Reference' string. Here, printing $str variable results 'This is Call By Reference'. It is because changes are done in the actual variable $str.

<?php  
function adder(&$str2)  
{  
    $str2 .= "Call By Reference";  
}  
$str = "This is";  
adder($str);  
echo $str;  
?>  

Output: This is Call By Reference

PHP Call By Reference Example 2

<?php  
function increment(&$i)  
{  
    $i++;  
}  
$i = 10;  
increment($i);  
echo $i;  
?>  

Output: 11