Slugify. Note: Requires PHP >= 5.4 and the Intl extension

functions.php

<?php
function slugify($string, $toLower = true) {
    $rules = "Any-Latin; Latin-ASCII; NFD; [:Nonspacing Mark:] Remove; NFC; [:Punctuation:] Remove;";
    if ($toLower) {
        $rules .= ' Lower();';
    }

    $string = transliterator_transliterate($rules, $string);

    // Remove repeating hyphens and spaces (e.g. 'foo---bar' becomes 'foo-bar')
    $string = preg_replace('/[-\s]+/', '-', $string);

    return trim($string, '-');
}