'word1', 'word2' => 'word2', 'word3' => 'word3' ); $text = "This is a text about Word1. We also talk about word2 and WORD3. Also woRd3, word2 and WORD1"; $pttrns = array_map(function ($val) {return "/" . preg_quote($val) . "/i"; },array_keys($glossary)); $res = preg_replace_callback($pttrns, function ($m) use($glossary){ return str_replace(strtolower($m[0]), $m[0], $glossary[strtolower($m[0])]); }, $text); echo $res;