issue as init.inc.php try to extend memory at no limit

hello
our PHPFox is under development, but in place in my server
I did not change parameters regarding memory , the line in init.inc.php is
@ini_set('memory_limit', '512M');

but my syslog file shows every few minutes some strange alert :

ALERT - script tried to disable memory_limit by setting it to a negative value -1 bytes which is not allowed (attacker *******) file '/var/www/********/include/init.inc.php', line 13)

any idea about what I should do ?
regards
Luc