Absolutely positive on the PHP version? You should check it from within XF via admin.php?tools/phpinfo . It's fairly common to have multiple PHP versions on a server and they tend to default to the older version.
You should contact your host and have them tell you how to switch to PHP 5.4 (or newer).
While PHP 5.2 is still supported for XF right now, we don't support downgrading to it. PHP 5.2 is completely unsupported by the PHP developers so I strongly recommend migrating to a newer version. There are likely unfixed security issues in the version you're using.
Depending on the host (if this is a shared hosting provider) you can frequently add some code to your .htaccess to change the PHP version if they offer multiple ones (HostGator does this). Generally the hosting provider FAQ will have that information in it.