Lazy.php

<?php

/**
 * Trait Lazy
 *
 * Allows for lazy initialization of object and class values.
 *
 * Example:
 *
 * class Test {
 *   use Lazy;
 *
 *   // The name of the functions, i.e. 'x' and 'y', will be used as a key for the storage of initialized values
 *
 *   function x() {  return $this->lazy(   function(){ return 'heavy-computation'; }   );  }
 *
 *   static function y() {  return self::lazyStatic(   function(){ return 'heavy-computation'; }   );  }
 * }
 */
trait Lazy
{
	private $__lazilyInitialized = [];
	private static $__lazilyInitializedStatic = [];

	private static function __initializeLazily(&$storage, callable $initializer)
	{
		$key = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 3)[2]['function'];

		if (!array_key_exists($key, $storage))
		{
			if (is_callable($initializer))
				$storage[$key] = call_user_func($initializer);
			else
				throw new RuntimeException('$initializer is not callable');
		}

		return $storage[$key];
	}

	/**
	 * @param callback $initializer
	 * @return mixed
	 */
	protected function lazy(callable $initializer)
	{
		return self::__initializeLazily($this->__lazilyInitialized, $initializer);
	}

	/**
	 * @param $initializer
	 * @return mixed
	 */
	protected static function lazyStatic(callable $initializer)
	{
		return self::__initializeLazily(self::$__lazilyInitializedStatic, $initializer);
	}
}

Run the function when needed and then store the result so the closure is not called a second time.