public function once( $key, $callback ) {
		if ( ! is_string( $key ) && ! is_callable( $callback ) ) {
			return;
		}

		if ( ! $this->exists( $key ) ) {
			$result = $callback();
			$this->singleton( $key, $result );
		}

		return $this->make( $key );
	}
public function once( $key, $callback ) {
		if ( ! is_string( $key ) && ! is_callable( $callback ) ) {
			return;
		}

		if ( ! $this->exists( $key ) ) {
			$result = $callback();
			$this->singleton( $key, $result );
		}

		return $this->make( $key );
	}
public function once ( $ key , $ callback ) { if ( ! is_string ( $ key ) && ! is_callable ( $ callback ) ) { return ; } if ( ! $ this -> exists ( $ key ) ) { $ result = $ callback ( ) ; $ this -> singleton ( $ key , $ result ) ; } return $ this -> make ( $ key ) ; }
Add key and value to the container once.

@param  string $key
@param  mixed  $callback

@return mixed
Add key and value to the container once .