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function is not described

_deprecated_function() WP 2.5.0

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

There is a {@see 'hook deprecated_function_run'} that will be called that can be used to get the backtrace up to what file and function called the deprecated function.

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

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

Hooks in function
Return

Nothing (null).

Usage

_deprecated_function( $function, $version, $replacement );
$function(string) (required)
The function that was called.
$version(string) (required)
The version of WordPress that deprecated the function.
$replacement(string)
The function that should have been called.
Default: null

Code of deprecated function: wp-includes/functions.php VER 5.1.1

<?php
function _deprecated_function( $function, $version, $replacement = null ) {

	/**
	 * Fires when a deprecated function is called.
	 *
	 * @since 2.5.0
	 *
	 * @param string $function    The function that was called.
	 * @param string $replacement The function that should have been called.
	 * @param string $version     The version of WordPress that deprecated the function.
	 */
	do_action( 'deprecated_function_run', $function, $replacement, $version );

	/**
	 * Filters whether to trigger an error for deprecated functions.
	 *
	 * @since 2.5.0
	 *
	 * @param bool $trigger Whether to trigger the error for deprecated functions. Default true.
	 */
	if ( WP_DEBUG && apply_filters( 'deprecated_function_trigger_error', true ) ) {
		if ( function_exists( '__' ) ) {
			if ( ! is_null( $replacement ) ) {
				/* translators: 1: PHP function name, 2: version number, 3: alternative function name */
				trigger_error( sprintf( __( '%1$s is <strong>deprecated</strong> since version %2$s! Use %3$s instead.' ), $function, $version, $replacement ) );
			} else {
				/* translators: 1: PHP function name, 2: version number */
				trigger_error( sprintf( __( '%1$s is <strong>deprecated</strong> since version %2$s with no alternative available.' ), $function, $version ) );
			}
		} else {
			if ( ! is_null( $replacement ) ) {
				trigger_error( sprintf( '%1$s is <strong>deprecated</strong> since version %2$s! Use %3$s instead.', $function, $version, $replacement ) );
			} else {
				trigger_error( sprintf( '%1$s is <strong>deprecated</strong> since version %2$s with no alternative available.', $function, $version ) );
			}
		}
	}
}