This would be tremendously useful, especially when the cause of the memory overrun is not the PHP code but rather due to the data input to the script.

For example: In Horde/IMP, we need to parse MIME e-mail messages.  These e-mail messages contain data that potentially could be unfiltered.  A particular mail message script MUST parse the entire data input in order to show anything (you cannot truncate MIME data, or else the resulting MIME message is invalid).  Even using things like temporary data streams, there is still a chance that the memory limit can be exceeded.

It would be tremendously useful to be able to define a memory limit exceeded handler for any given script that could catch the case where we can not parse the message due to a memory limit caused by the message data itself.  This handler could allow the script to do something like automatically redirect the browser to a different page and display an error message.

That is rather tricky because such a callback could continue consuming memory. We 
would have to introduce the concept of soft and hard limits or something along 
those lines for this to be realistic.

Oh, and why wouldn't you just use a shutdown function for this? If you do all 
your processing before sending any output and then check for an out of memory in 
the shutdown function you can easily do this redirect in this case.