I forget now where I got this, probably stack overflow somewhere, but it's been in my library of common functions for a while. IIRC it was based on the wordpress slugify or maybe drupal or something.

slugify.php

function slugify($text) {
  
		// replace non letter or digits by -
		$text = preg_replace('~[^\\pL\d]+~u', '-', $text);
	
		// trim
		$text = trim($text, '-');
	
		// transliterate
		$text = iconv('utf-8', 'us-ascii//TRANSLIT', $text);
	
		// lowercase
		$text = strtolower($text);
	
		// remove unwanted characters
		$text = preg_replace('~[^-\w]+~', '', $text);
	
		if (empty($text))
		{
			return 'n-a';
		}
	
		return $text;
	
	}