If there is a system account, make sure you change the shell to /sbin/nologin or /bin/false. In order for the changes to take effect, run

semodule -B

In order to overcome the error without modifying the server side php.ini file, you need to use the ini_set function to increase the memory limit inserting the following code in the php file itself. Keep on increasing the memory limit unless the error disappears.

ini_set(”memory_limit”,”64M”);

First download and extract mod_security
cd /etc/httpd/
wget http://www.modsecurity.org/download/modsecurity-apache_2.5.7.tar.gz
tar -zxf modsecurity-apache_2.5.7.tar.gz
cd modsecurity-apache_2.5.7/apache2

Next compile mod_security at a module using apxs. BTW, APXS is an Apache tool for building Apache modules.
apxs -cia mod_security2.c

you may realise you don’t have the apxs on your server, in that case you will have to install httpd-devel package using yum.
yum install httpd-devel

Once mod_security2 is installed, make sure you have the ‘LoadModule‘ line in your Apache configuration (httpd.conf) file and the module “mod_security2.so” itself under ‘modules’ directory.

LoadModule security2_module modules/mod_security2.so

Now, create a modsecurity configuration file as modsecurity.conf under /etc/httpd/conf.d/ directory, as *.conf in the conf.d directory gets loaded. Below is the sample configuration file you may use:

#SecFilterEngine DynamicOnly
SecFilterEngine On
SecFilterDefaultAction “deny,log,status:500″
SecFilterScanPOST On
SecFilterCheckURLEncoding On
SecFilterCheckCookieFormat On
SecFilterCheckUnicodeEncoding Off
SecFilterNormalizeCookies On
SecFilterCookieFormat 1
SecServerResponseToken Off

#If you want to scan the output, uncomment these
#SecFilterScanOutput On
#SecFilterOutputMimeTypes “(null) text/html text/plain”

# Only record the interesting stuff
SecAuditEngine RelevantOnly
SecAuditLog /var/log/modsecurity/audit_log

# You normally won’t need debug logging
SecFilterDebugLevel 0
SecFilterDebugLog /var/log/modsecurity/modsec_debug_log

#Include the sample rule files, you just downloaded
Include /etc/httpd/conf.d/apache2-rules.conf*
Include /etc/httpd/conf.d/badips.conf*
Include /etc/httpd/conf.d/blacklist.conf*
Include /etc/httpd/conf.d/blacklist2.conf*
Include /etc/httpd/conf.d/proxy.conf*
Include /etc/httpd/conf.d/rootkits.conf*
Include /etc/httpd/conf.d/rules.conf*

Download the sample rules from
wget http://hostanswers.net/modsecurity/sample_mod_rules.tar.gz

Please note: These are just the default rules that come with Mod_security2 and you need to modify them as per your needs.

At the end, make sure you create a modsecurity directory and ‘audit_log’ file and restart apache.
mkdir /var/log/modsecurity
touch /var/log/modsecurity/audit_log
service httpd restart

If you have problems accessing websites look at error log at
/etc/httpd/logs/audit_log

Just FYI, if you wish to disable mod_security, just comment the modsecurity ‘LoadModule’ line in your Apache configuration file and restart apache.