PHP Fatal error: Allowed memory size of 536870912 bytes exhausted

I have this kind of error on the CI 3.1.5 which I recently migrated my legacy 2.2.x to and I tried to increase it 
by adding 
   ini_set('memory_limit', '1024M') where I found the problem but still happened

Anyone had the same problem? And how do you fix it? 

Greatly appreciate 

PS: the old legacy runs fine and never has this problem!

Sounds like you have an endless loop or bad code,
check your code.

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )

You might not be allowed by the server setting to change php.ini values in code.
Try changing it directly in the php.ini

On the package it said needs Windows 7 or better. So I installed Linux.

Agree with insiteFX, sounds like you have a coding problem somewhere. Unless you are running a truly massive or complex scenario, in which case you might need to rethink how you are approaching the whole thing.

I had this with a pdf library once which had me flumoxed. Change libraries and everything was fine again.

Thanks for all suggestions, I dont think I have endless loop as the same code runs fine under 2.2.x and I migrated to CI 3.1.5
with little changes except class names for my custom controllers/ libraries/ models. I made some changes today and try again to see if this is gone away.

I did use mpdf as libraries as mine is heavily computing data and make pdf output for deliveries but I dont get the point if this
is the contributing factor to the problem. I have multiple users access to app but not like 100s at the same time so on...

Regards