I have mailed atleast a dozen people with visible PHP errors on their website and a convincing reason that I am capable of helping them out but never got a response. Is there anything else you suggest that might work?
The PHP errors may not be enough on their own to convince them that there's a problem worth fixing.
You need to tie those errors to business problems/benefits. A client may not know / care what a php error does to their site unless they know that it's losing them money, conversions, visitors, etc.
