The behaviour of these functions is affected by settings in php.ini.
Here's a short explanation of the configuration directives.
Disables or enables the extension. Default value is 0, i.e. disabled.
Soft memory limit.
The extension checks memory consumption before and after executing an op_array and produces a warning is the difference between the two values is equal to or greater than the soft limit, but only if the function is not ignored.
Setting this option to 0 also disables both soft and hard limit warnings. Default value is 0, i.e. no warnings is produced.
Hard memory limit.
The extension checks memory consumption before and after executing an op_array and produces a warning is the difference between the two values is equal to or greater than the hard limit, even if the function is ignored. Setting this option to 0 disables hard limit warnings completely. Default value is 0, i.e. no hard limit warnings is produced.
Virtual memory limit (set on a process).
This limit is checked only on shutdown and a warning is produced if the value is greater than or equal to the limit.
This option is currently supported only on OSes where mallinfo() function is available (i.e. Linux).
A comma or whitespace-separated list of functions which are to be ignored by soft_limit. The values are case-insensitive, for class methods use class::method syntax.