undefined

Goddamn this undefined error is getting on my nerves. It doesn't matter what I put where to catch the error it still pops up. For ages I wasn't even sure what was generating it. PHP? Javascript? Today the finger was potentially pointed at JQuery Mobile. I think I've isolated it to a single PHP statement that shouldn't be causing any problems because the objective of this statement is to catch undefined variables!! This is driving me nuts, I really don't have the time.
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I just figured it out. It IS JQuery Mobile. It didn't like a function that is supposed to redirect to a page on an error. I've worked around it by taking the function away and putting a html message and okay button which links back to the page instead. Seems redundant but at least I can get on with things. This error will be scattered all over the place because I did a marathon session a while ago and implemented it everywhere that it would be needed. I just need to change everywhere now.