Strict Standards will be thrown out if you put exploded array in array_pop:
<?php
$a = array_pop(explode(",", "a,b,c"));
echo $a;
?>

You will see:
PHP Strict Standards:  Only variables should be passed by reference in - on line 2

Strict Standards: Only variables should be passed by reference in - on line 2
c

Notice that, you should assign a variable for function explode, then pass the variable reference into array_pop to avoid the Strict Standard warning.