Syntax

apply_filters(string $tag, mixed $value)

Usage

This is a callback function that has to been added to a filter hook.  The callback functions attached to the filter hook are invoked by calling this function. apply_filters() function can be used to create a new filter hook by simply calling this function with the name of the new hook specified using the $tag parameter.
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;
}