src/core/class-papi-core-container.php
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