Basic php: exec() question

Yii 2.0 General Discussions
1 
	public function execute($cmd)
	{
		// Some commands fail without sending output to cmdBuff.
		// This captures them to a text file in the task dir
		$fnErr = $this->taskDir . '_stderr.txt';
		$rc=0;$this->cmdBuff=[];
		file_put_contents($fnErr, PHP_EOL . $cmd . PHP_EOL, FILE_APPEND);
		exec("$cmd 2>>$fnErr", $this->cmdBuff, $rc);
		return $rc;
	}

I am unhappy with this kludge.

“$cmd 2>&1” is not good enough. It is not capturing everything.
I hoped maybe some merciful guru here might know the better way…