You can use regular expression to get the desired result.
$str = preg_replace('/[^a-z\d ]/i', '', $str);
The i stands for case insensitive. ^ means, does not start with. \d matches any digit. a-z matches all characters between a and z. Because of the i parameter you don't have to specify a-z and A-Z. After \d there is a space, so spaces are allows in this regex.