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function is not described

_deprecated_file() WP 2.5.0

Mark a file as deprecated and inform when it has been used.

There is a hook deprecated_file_included that will be called that can be used to get the backtrace up to what file and function included the deprecated file.

The current behavior is to trigger a user error if WP_DEBUG is true.

This function is to be used in every file that is deprecated.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

Return

null. Nothing.

Usage

_deprecated_file( $file, $version, $replacement, $message );
$file(string) (required)
The file that was included.
$version(string) (required)
The version of WordPress that deprecated the file.
$replacement(string)
The file that should have been included based on ABSPATH.
Default: ''
$message(string)
A message regarding the change.
Default: ''

Changelog

Since 2.5.0 Introduced.
Since 5.4.0 This function is no longer marked as "private".
Since 5.4.0 The error type is now classified as E_USER_DEPRECATED (used to default to E_USER_NOTICE).

Code of _deprecated_file() WP 5.8.1

function _deprecated_file( $file, $version, $replacement = '', $message = '' ) {

	/**
	 * Fires when a deprecated file is called.
	 *
	 * @since 2.5.0
	 *
	 * @param string $file        The file that was called.
	 * @param string $replacement The file that should have been included based on ABSPATH.
	 * @param string $version     The version of WordPress that deprecated the file.
	 * @param string $message     A message regarding the change.
	 */
	do_action( 'deprecated_file_included', $file, $replacement, $version, $message );

	/**
	 * Filters whether to trigger an error for deprecated files.
	 *
	 * @since 2.5.0
	 *
	 * @param bool $trigger Whether to trigger the error for deprecated files. Default true.
	 */
	if ( WP_DEBUG && apply_filters( 'deprecated_file_trigger_error', true ) ) {
		$message = empty( $message ) ? '' : ' ' . $message;

		if ( function_exists( '__' ) ) {
			if ( $replacement ) {
				trigger_error(
					sprintf(
						/* translators: 1: PHP file name, 2: Version number, 3: Alternative file name. */
						__( '%1$s is <strong>deprecated</strong> since version %2$s! Use %3$s instead.' ),
						$file,
						$version,
						$replacement
					) . $message,
					E_USER_DEPRECATED
				);
			} else {
				trigger_error(
					sprintf(
						/* translators: 1: PHP file name, 2: Version number. */
						__( '%1$s is <strong>deprecated</strong> since version %2$s with no alternative available.' ),
						$file,
						$version
					) . $message,
					E_USER_DEPRECATED
				);
			}
		} else {
			if ( $replacement ) {
				trigger_error(
					sprintf(
						'%1$s is <strong>deprecated</strong> since version %2$s! Use %3$s instead.',
						$file,
						$version,
						$replacement
					) . $message,
					E_USER_DEPRECATED
				);
			} else {
				trigger_error(
					sprintf(
						'%1$s is <strong>deprecated</strong> since version %2$s with no alternative available.',
						$file,
						$version
					) . $message,
					E_USER_DEPRECATED
				);
			}
		}
	}
}