Contao\CoreBundle\Config\Dumper\CombinedFileDumper::setHeader PHP Method

    public function setHeader($header)
    {
        if (0 !== strpos($header, '<?php')) {
            throw new \InvalidArgumentException('The file header must start with an opening PHP tag.');
        }
        $this->header = $header;
    }

 /**
  * Tests setting an invalid header.
  *
  * @expectedException \InvalidArgumentException
  */
 public function testInvalidHeader()
 {
     /** @var Filesystem|\PHPUnit_Framework_MockObject_MockObject $filesystem */
     $filesystem = $this->getMock('Symfony\\Component\\Filesystem\\Filesystem');
     /** @var PhpFileLoader|\PHPUnit_Framework_MockObject_MockObject $fileLoader */
     $fileLoader = $this->getMock('Contao\\CoreBundle\\Config\\Loader\\PhpFileLoader');
     $dumper = new CombinedFileDumper($filesystem, $fileLoader, $this->getCacheDir());
     $dumper->setHeader('No opening PHP tag');
 }