Create a short, fairly unique, urlsafe id string for a given input. This uses (almost) all allowed urlsafe characters to maximize the possible number of values.

generate_id.php

/**
 * Create a short, fairly unique, urlsafe hash for the input string.
 */
function generate_id( $input, $length = 8 ){
	// Create a raw binary sha256 hash and base64 encode it.
	$hash_base64 = base64_encode( hash( 'sha256', $input, true ) );
	// Replace non-urlsafe chars to make the string urlsafe.
	$hash_urlsafe = strtr( $hash_base64, '+/', '-_' );
	// Trim base64 padding characters from the end.
	$hash_urlsafe = rtrim( $hash_urlsafe, '=' );
	// Shorten the string before returning.
	return substr( $hash_urlsafe, 0, $length );
}