<?php
use warnings;
$glossary = array(
    'word1' => '<span title="Explanation for Word 1">word1</span>',
    'word2' => '<span title="Explanation for Word 2">word2</span>',
    'word3' => '<span title="Explanation for Word 3">word3</span>'
);
 
$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;

PD9waHAKdXNlIHdhcm5pbmdzOwokZ2xvc3NhcnkgPSBhcnJheSgKICAgICd3b3JkMScgPT4gJzxzcGFuIHRpdGxlPSJFeHBsYW5hdGlvbiBmb3IgV29yZCAxIj53b3JkMTwvc3Bhbj4nLAogICAgJ3dvcmQyJyA9PiAnPHNwYW4gdGl0bGU9IkV4cGxhbmF0aW9uIGZvciBXb3JkIDIiPndvcmQyPC9zcGFuPicsCiAgICAnd29yZDMnID0+ICc8c3BhbiB0aXRsZT0iRXhwbGFuYXRpb24gZm9yIFdvcmQgMyI+d29yZDM8L3NwYW4+JwopOwoKJHRleHQgPSAiVGhpcyBpcyBhIHRleHQgYWJvdXQgV29yZDEuIFdlIGFsc28gdGFsayBhYm91dCB3b3JkMiBhbmQgV09SRDMuIEFsc28gd29SZDMsIHdvcmQyIGFuZCBXT1JEMSI7CgokcHR0cm5zID0gYXJyYXlfbWFwKGZ1bmN0aW9uICgkdmFsKSB7cmV0dXJuICIvIiAuIHByZWdfcXVvdGUoJHZhbCkgLiAiL2kiOyB9LGFycmF5X2tleXMoJGdsb3NzYXJ5KSk7CiRyZXMgPSBwcmVnX3JlcGxhY2VfY2FsbGJhY2soJHB0dHJucywgZnVuY3Rpb24gKCRtKSB1c2UoJGdsb3NzYXJ5KXsKCXJldHVybiBzdHJfcmVwbGFjZShzdHJ0b2xvd2VyKCRtWzBdKSwgJG1bMF0sICRnbG9zc2FyeVtzdHJ0b2xvd2VyKCRtWzBdKV0pOwp9LCAkdGV4dCk7CmVjaG8gJHJlczs=