We needed to locate where most of the memory is allocated in our scripts (as some of them became too memory hungry), so I've created memtrack extension.
This extension helps us to see unusually big memory allocations in production code.

Of course we use error logging, so memtrack just produces warnings like this one:
Warning: [memtrack] [pid 26177] user function foo() executed in /tmp/1.php on line 7 allocated 4194304 bytes

[memtrack] is a prefix for all warnings produced by the extension, so that one could find them in the logs.
[pid 26177] is, well, a PID, so that one could follow a certain process (if needed).
user function means it's a user defined function; the ones provided by PHP or one of its extension are reported as internal functions.

Memtrack supports the following INI options at the moment:

• memtrack.enabled - Boolean flag, which enables or disabled the extension. Default value is 0, i.e. disabled.
  
• memtrack.soft_limit - "Soft" memory limit. It means that a certain function cannot increase memory usage by more than X bytes and a warning is produced when the limit is exceeded, if the function is not ignored. Default value is 0.
  
• memtrack.hard_limit - "Hard" memory limit. A warning is produced when the limit is exceeded, whether or not the function is listed in memtrack.ignore_functions. Default value is 0.
  
• memtrack.ignore_functions - A comma- or whitespace-separated list of functions, which are to be ignored. The values are case-insensitive, for class methods use Class::method syntax. Empty by default.
  
• memtrack.vm_limit - Virtual memory size limit (set on a process), measured on shutdown only, might be helpful if you suspect a process is "leaking" memory. A warning is produced when the limit is reached. This option is currently available only on Linux (if you know how to get virtual memory size of the current process on any other system - please let me know). Default value is 0.
  

The extension checks memory usage changes per op_array, which in most cases means a function call or an included file with flat code, so you should not notice any performance loss.

memtrack is still BETA, so let me know if you find any problems there, or if you have any suggestions on how to improve it.
Pierre is working on Win32 build just as I write this, so Win32 dlls should be available soon, stay tuned.