src/Payum/Core/Extension/ExtensionCollection.php
public function addExtension(ExtensionInterface $extension, $forcePrepend = false)
    {
        $forcePrepend ?
            array_unshift($this->extensions, $extension) :
            array_push($this->extensions, $extension)
        ;
    }
public function addExtension(ExtensionInterface $extension, $forcePrepend = false)
    {
        $forcePrepend ?
            array_unshift($this->extensions, $extension) :
            array_push($this->extensions, $extension)
        ;
    }
@param ExtensionInterface $extension
@param bool               $forcePrepend

@return void