PHP 5.3.3 isn’t insecure or unsupported when talking about an LTS release, such as what was likely RHEL or CentOS 6. When LTS linux distributions release versions like this, they are researching each and every new CVE that gets reported for the software and patch the security issues.

RHEL/CentOS 6 will continue to support (i.e. customer support, security and critical bug fixes) PHP 5.3.3 (ironically if using php-fpm its 5.3.18) for a good few years from now (2020 with 3 more years RHEL extended support), probably becoming EOL after php.net EOL’s PHP 7.0.