You need to escape your quotes, or use doubles, or use heredoc:
// Escaped
return '<div id="xxxx-xxxxxx"></div>
<script type="text/javascript">
window._taboola = window._taboola || [];
_taboola.push({
mode: \'xxxxxxx-xxxxx\',
container: \'xxxxxx-xxxxxxx\',
placement: \'one word word\',
target_type: \'mix\'
});
</script>';
// Doubles
return '<div id="xxxx-xxxxxx"></div>
<script type="text/javascript">
window._taboola = window._taboola || [];
_taboola.push({
mode: "xxxxxxx-xxxxx",
container: "xxxxxx-xxxxxxx",
placement: "one word word",
target_type: "mix"
});
</script>';
// Heredoc
return <<<html
<div id="xxxx-xxxxxx"></div>
<script type="text/javascript">
window._taboola = window._taboola || [];
_taboola.push({
mode: "xxxxxxx-xxxxx",
container: "xxxxxx-xxxxxxx",
placement: "one word word",
target_type: "mix"
});
</script>
html;