// Created on savesnippets.com · https://savesnippets.com/e6pUtl9Te5XzAd
<?php
function highlightTerm(string $text, string $term): string {
    if ($term === '') return htmlspecialchars($text, ENT_QUOTES);
    $escapedText = htmlspecialchars($text, ENT_QUOTES);
    $escapedTerm = htmlspecialchars($term, ENT_QUOTES);
    $pattern     = '/(' . preg_quote($escapedTerm, '/') . ')/iu';
    return preg_replace($pattern, '<mark>$1</mark>', $escapedText);
}

echo highlightTerm("PHP is great, PHP is fun.", "php");
// <mark>PHP</mark> is great, <mark>PHP</mark> is fun.