MODIFICATION KOD COUOP

Hello i want have codes coupons OPEN 0001 to OPEN 5000. The first I start edit code but it’s not work. Can you help me add space after the word “open”

function wccg_get_random_coupon() {

	// Generate unique coupon code
	
	$random_coupon = 'OPEN';
	$length        = 4;
	$charset       = '0123456789';
	$count         = strlen( $charset );

	while ( $length-- ) {
		$random_coupon .= $charset[ mt_rand( 0, $count-1 ) ];
	}

	$random_coupon = implode( '', str_split( strtoupper( $random_coupon ), ) );

	// Ensure coupon code is correctly formatted
	$coupon_code = apply_filters( 'woocommerce_coupon_code', $random_coupon );

	return $coupon_code;
}