<?php
$name = "abc";

$counter = 1;
if($pos = strrpos($name, '_')) {
    $oldcounter = substr($name, $pos + 1);
    if(is_numeric($oldcounter)){
        $counter = $oldcounter + 1;
        $name = substr($name, 0, $pos);
    }
}

$newname = $name . '_' . $counter;

print $newname;
?>