the php dont request the needed memory, it request for the next block. So:
When i has 256M it request another ±20M.
Then I increase to 512M. Then the system claims for another 20M.
Then I increase to 1024M and then the system claims for another 20M
I repeat the actions to accure memory size to 1536M.
If it’s necesary to increase the memory a lot then maybe it’s necesary increase the TimeOut and ProxyTimeout httpd.conf paramenters.