Syntax

add_filter( string $tag, callable $function_to_add, int $priority = 10, int $accepted_args = 1)

Usage

It is called by themes/plugins, is used to add filters to the queue to be applied to the hook by core.
Add this below code in wp-includes/plugin.php

function apply_filters( $tag, $value ) {
    global $wp_filter, $wp_current_filter;
    $args = func_get_args();
    if ( isset( $wp_filter['all'] ) ) {
        $wp_current_filter[] = $tag;
        _wp_call_all_hook( $args );
    }
    if ( ! isset( $wp_filter[ $tag ] ) ) {
        if ( isset( $wp_filter['all'] ) ) {
            array_pop( $wp_current_filter );
        }
        return $value;
    }
    if ( ! isset( $wp_filter['all'] ) ) {
        $wp_current_filter[] = $tag;
    }
    array_shift( $args );
    $filtered = $wp_filter[ $tag ]->apply_filters( $value, $args );
    array_pop( $wp_current_filter );
    return $filtered;
}