It fills between two text with the given character until its length reach the given limit.

str_pad_center.php

<?php
function str_pad_center($prefix, $suffix, $length, $char) {
	$countWithoutChar = strlen($prefix) + strlen($suffix);
	
	if($countWithoutChar > $length) {
		return ""; // In your case, you may want to change this
	}
	
	$fill = str_repeat($char, $length - $countWithoutChar);
	
	return $prefix . $fill . $suffix;
}

echo str_pad_center("PR", "CO", 15, "_"); // PR___________CO / Length: 15
echo str_pad_center("PR", "CO", 10, "_"); // PR______CO / Length: 10

I turn this into a JavaScript library: https://github.com/ozgurg/pad-center