Update input.php

The original code 
$attrSubSet[0] = array_pop(explode(' ', trim($attrSubSet[0]))); 
throws an Strict standards: Only variables should be passed by reference 
In array_pop the only parameter is an array passed by reference. The return value does not have any reference.
Merge pull request #2489 from avjoomla/patch-1

Update input.php