[UPDATE: found out the real problem]
Recently, we had to move an application from the staging environment to pre-production ( where stage means we show it to the customer and pre-production is the customer hosts it waiting for the go-live ) without the possibility of replicating the whole VM due to incompatibility issues for the VM conversion ( VMware to Xen, if I correctly rembember ).
After setting up the new environment we experienced serious issue on the search engine and other minor components related with GET and POST parameters: after a while we compared the installations of PHP to find out the problem and it came out that the PCRE extension used in the new environment ( CentOS, if I – again – correctly remember ) was released in 2006 ( version 6.something ).
You can check it from the phpinfo() looking for:
- PCRE (Perl Compatible Regular Expressions) Support
- PCRE Library Version
After updating ( version 7.8 ) it everything got normal.