I guess the level of security is how well the server is configured (php.ini file, apache whatever file etc). And if the server is behind firewalls and a coupple of routers.... well, I would assume the source code to be pretty secure.
Here's some threads on security issues.
http://www.phpbuilder.com/board/showthread.php?s=&postid=10372438#post10372438
http://www.phpbuilder.com/board/showthread.php?s=&threadid=10245425
And other reasons:
PHP is FREE! (contra asp and .net)
PHP has good technical support
Great comunity for help (like here)
It's a fast growing ad hoc standard
It's extremely easy to learn (unlike .net and asp)
and did I mention that it is FREE?
If upper management tells you it is insecure, ask them to elaborate on that. Can they document it? Because if they know it, they must have gotten the information from somewhere.
And if I'm not mistaken, even HIGHER management allways counts nickles and dimes. Just show them how much they will be saving.
e.g. asp.net with oracle vs PHP with MySql.