// $foo is de zin en $array is de array met keyvalues
function explodeSentence($foo, $array) {
    $sntc = "";
    strtolower($foo);
    $str = explode(" ", $foo);

    foreach($str as $word) {
        if($array[$word] > 1) {
            $sntc .= "<span style='background-color: yellow;'>$word</span> ";
        } else {
            $sntc .= $word;
        }
    }

    return $sntc;
}