<?php
 
$dot_prod = "at the coast will reach the Douglas County coast";
$bold = array("coast", "the", "pass");
$unbold = array("county coast", "at the", "grants pass");
usort($unbold, function($a, $b) { return strlen($b) - strlen($a); });
usort($bold, function($a, $b) { return strlen($b) - strlen($a); });
$rx = "~\b(?:" . implode("|", $unbold) . ")\b(*SKIP)(*F)|\b(?:" . implode("|", $bold) . ")\b~i";
echo "$rx\n";
echo preg_replace($rx, "<b>$0</b>", $dot_prod);

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