Examples of implementing helper methods for WPLib as instance methods (vs. static methods)

1-example.php

<?php 

if ( WPLib::is_development() ) {

	define( 'WP_DEBUG', true );

} else {

	define( 'WP_DEBUG', false );

}

2-class-wplib-workflow.php

<?php


class _WPLib_Workflow {

	var $runmode = 'dev';

	static function on_load() {

		WPLib::register_helper( __CLASS__ );

	}

	/**
	 * @return bool
	 *
	 * @static   <------ THIS IS WHAT I NEED PHPSTORM TO RECOGNIZE
	 */
	function is_development() {

		return 'dev' === $this->runmode;

	}

}
_WPLib_Workflow::on_load();

3-wplib.php

<?php


class WPLib {

	/**
	 * Register a "helper" method for this the WPLib class, or another class.
	 *
	 * @param string $helper_class
	 * @param string $class_to_help
	 */
	public static function register_helper( $helper_class, $class_to_help = 'WPLib' ) {

		WP_Helper::_register_helper( $helper_class, $class_to_help );

	}

	/**
	 * Magic method that checks for "helper" methods when static calls are made.
	 *
	 * @param string $helper_method
	 * @param array $args
	 *
	 * @return mixed
	 */
	public static function __callStatic( $helper_method, $args ) {

		return WP_Helper::_call_helper( get_called_class(), $helper_method, $args );

	}

}

4-class-wplib-helper.php

<?php

/**
 * Class WPLib_Helper - Provides "Helper" functionality for WPLib and maybe other classes.
 */
class WPLib_Helper {

	/**
	 * List of helper classes for the WPLib class
	 *
	 * @var array
	 */
	private static $_helpers;

	/**
	 * Registers helper methods for WPLib or another class.
	 *
	 * @param string $helper_class
	 * @param string $class_to_help
	 */
	static function _register_helper( $helper_class, $class_to_help  ) {

		$methods = array_fill_keys( array_filter( get_class_methods( $helper_class ),
			function( $element ) {
				return '__' !== substr( $element, 0, 2 );
			} ),
			new $helper_class()
		);

		unset( $methods[ 'on_load' ] );

		if ( ! isset( self::$_helpers[ $class_to_help ] ) ) {

			self::$_helpers[ $class_to_help ] = $methods;

		} else {

			self::$_helpers[ $class_to_help ] += $methods;

		}

	}

	/**
	 * Calls registered helper methods for the class being helped.
	 *
	 * @param string $class_helped
	 * @param string $helper_method
	 * @param array $args
	 * @return mixed
	 */
	static function _call_helper( $class_helped, $helper_method, $args ) {

		if ( isset( self::$_helpers[ $class_helped ][ $helper_method ] ) ) {

			return call_user_func_array( array( self::$_helpers[ $class_helped ][ $helper_method ], $helper_method ), $args );

		} else {

			$message = 'There is no helper method %s registered for class %s.';

			trigger_error( sprintf( $message, $helper_method, $class_helped ), E_USER_ERROR );

			return null;

		}

	}

}