Incrementing alphanumeric strings in PHP

alphanumeric_increment.php

function alphanumeric_increment( $string, $position=false ) {
        if ( false === $position ) {
                $position = strlen( $string ) - 1;
        }
        $increment_str = substr( $string, $position, 1 );
        switch ( $increment_str ) {
                case '9':
                        $string = substr_replace( $string, 'a', $position, 1 );
                        break;
                case 'z':
                        $string = substr_replace( $string, 'A', $position, 1 );
                        break;
                case 'Z':
                        if ( 0 === $position ) {
                                $string = substr_replace( $string, '0', $position, 1 );
                                $string .= '0';
                        } else {
                                $inc_position = $position - 1;
                                $string = increment( $string, $inc_position );
                                $string = substr_replace( $string, '0', $position, 1 );
                        }
                        break;
                default:
                        $increment_str++;
                        $string = substr_replace( $string, $increment_str, $position, 1 );
                        break;
        }
        return $string;
}

Line 19, should be:

$string = alphanumeric_increment( $string, $inc_position );

Function increment is not defined.