PHP webshell showing 127 status code for most of the commands


I have came across a site where I’m able to upload a PHP web shell. the problem I’m facing is that most of the commands I’m trying to run shows 127 status code. The only commands which I can run are echo, set, cd, pwd. What could be reason for this? Is there some kind of protection available in php which limits the capability of uploaded web shell? Through phpinfo() I was able to find that it has open_basedir set but does that also limits web shell from running normal commands? also it is using zend engine.