Source

File: wp-includes/class-wp-hook.php

public function apply_filters( $value, $args ) {
		if ( ! $this->callbacks ) {
			return $value;
		}

		$nesting_level = $this->nesting_level++;

		$this->iterations[ $nesting_level ] = array_keys( $this->callbacks );
		$num_args                           = count( $args );

		do {
			$this->current_priority[ $nesting_level ] = current( $this->iterations[ $nesting_level ] );
			$priority                                 = $this->current_priority[ $nesting_level ];

			foreach ( $this->callbacks[ $priority ] as $the_ ) {
				if ( ! $this->doing_action ) {
					$args[0] = $value;
				}

				// Avoid the array_slice() if possible.
				if ( 0 == $the_['accepted_args'] ) {
					$value = call_user_func( $the_['function'] );
				} elseif ( $the_['accepted_args'] >= $num_args ) {
					$value = call_user_func_array( $the_['function'], $args );
				} else {
					$value = call_user_func_array( $the_['function'], array_slice( $args, 0, (int) $the_['accepted_args'] ) );
				}
			}
		} while ( false !== next( $this->iterations[ $nesting_level ] ) );

		unset( $this->iterations[ $nesting_level ] );
		unset( $this->current_priority[ $nesting_level ] );

		$this->nesting_level--;

		return $value;
	}