- preg_match: documentation ( source)
<?php
$string = '<p align="center"><font face="Proxima Nova Alt Th" size="21" color="#ffffff" letterSpacing="-1.000000" kerning="1"><b>YOUR</b></font></p><p align="center"><font face="Proxima Nova Alt Th" size="21" color="#ffffff" letterSpacing="-1.000000" kerning="1"><b>OFFER</b></font></p>';
$regex = "/font face=\"([a-z 0-9]+)\"/i";
$font = preg_match($regex, $string, $matches);
echo $matches[1];