Validando el campo NIF o CIF en WooCommerce

//Validando el campo NIF/CIF
function validando_campo() {
	$falso = true;

    if ( isset( $_POST['billing_nif'] ) ) {
		$nif = strtoupper( $_POST['billing_nif'] );

		for ( $i = 0; $i < 9; $i ++ ) {
			$num[$i] = substr( $nif, $i, 1 );
		}

		if ( !preg_match( '/((^[A-Z]{1}[0-9]{7}[A-Z0-9]{1}$|^[T]{1}[A-Z0-9]{8}$)|^[0-9]{8}[A-Z]{1}$)/', $nif ) ) { //No tiene formato válido
			$falso = true;
		}

		if ( preg_match( '/(^[0-9]{8}[A-Z]{1}$)/', $nif ) ) {
			if ( $num[8] == substr( 'TRWAGMYFPDXBNJZSQVHLCKE', substr( $nif, 0, 8 ) % 23, 1 ) ) { //NIF válido
				$falso = false;
			}
		}

		$suma = $num[2] + $num[4] + $num[6];
		for ( $i = 1; $i < 8; $i += 2 ) {
			$suma += substr( ( 2 * $num[$i] ), 0, 1 ) + substr( ( 2 * $num[$i] ), 1, 1 );
		}
		$n = 10 - substr( $suma, strlen( $suma ) - 1, 1 );

		if ( preg_match( '/^[KLM]{1}/', $nif ) ) { //NIF especial válido
			if ( $num[8] == chr( 64 + $n ) ) {
				$falso = false;
			}
		}

		if ( preg_match( '/^[ABCDEFGHJNPQRSUVW]{1}/', $nif ) ) {
			if ( $num[8] == chr( 64 + $n ) || $num[8] == substr( $n, strlen( $n ) - 1, 1 ) ) { //CIF válido
				$falso = false;
			}
		}

		if ( preg_match( '/^[T]{1}/', $nif ) ) {
			if ( $num[8] == preg_match( '/^[T]{1}[A-Z0-9]{8}$/', $nif ) ) { //NIE válido (T)
				$falso = false;
			}
		}
		if ( preg_match( '/^[XYZ]{1}/', $nif ) ) { //NIE válido (XYZ)
			if ( $num[8] == substr( 'TRWAGMYFPDXBNJZSQVHLCKE', substr( str_replace( array( 'X','Y','Z' ), array( '0','1','2' ), $nif ), 0, 8 ) % 23, 1 ) ) {
				$falso = false;
			}
		}
	}

	if ( $falso ) {
		wc_add_notice( __( 'Por favor, introduzca un NIF/CIF válido.' ), 'error' );
	}
}
add_action( 'woocommerce_checkout_process', 'validando_campo' );