Ini_set / ini_get memory_limit php memory recovery

I am writing a script where I need to set a memory limit for something very high at a certain point. But after that I need to restore the memory limit.

But ini_get returns -1 if your script did not call ini_set to set a memory limit.

I am trying to do something like this:

$oldLimit  = ini_get("memory_limit");
ini_set("memory_limit", "220M");
do something 
//restore memory
ini_set("memory_limit", -1);

But $ oldLimit is -1.

which means -1 in this scenario.

Thanks in advance.