is_protected_meta( string $meta_key, string $meta_type = '' )

Source

File: wp-includes/meta.php

function is_protected_meta( $meta_key, $meta_type = '' ) {
	$sanitized_key = preg_replace( "/[^\x20-\x7E\p{L}]/", '', $meta_key );
	$protected     = strlen( $sanitized_key ) > 0 && ( '_' === $sanitized_key[0] );

	/**
	 * Filters whether a meta key is considered protected.
	 *
	 * @since 3.2.0
	 *
	 * @param bool   $protected Whether the key is considered protected.
	 * @param string $meta_key  Metadata key.
	 * @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
	 *                          or any other object type with an associated meta table.
	 */
	return apply_filters( 'is_protected_meta', $protected, $meta_key, $meta_type );
}