- preg_match: documentation ( source)
- str_replace: documentation ( source)
<?php
$string = "2017 MODEL YEAR 6.7L POWER STROKE V8 DIESEL 6-SPEED AUTO TRANS 3.55 ELECTRONIC LOCKING AXLE FX4 OFF-ROAD PACKAGE .SKID PLATES 10000-lb. GVWR 20000-lb PACKAGE";
Preg_match("/(\d+)(\d{3})-lb/ui", $string, $match);
$find = $match[0];
$replace = $match[1] .",". $match[2] ."-lb";
Echo length($match);
Echo str_replace($find, $replace, $string);