When using ZWAMP and running a non-trivial PHP script, the said error might occur several times. This is because the APC (which is enabled by default in ZWAMP) runs out of space when caching.
To fix this, go to the folder vdrive\.sys\php\php.ini in your ZWAMP folder, and look for the following code:
apc.shm_size=4M
Change 4M to something like 12M.