Tracked extensively under CVE-2019-9023 , the vulnerability resides within the underlying regular expression compilation engine ( compile_string_node , match_at , and fetch_token ).
Modern PHP offers better type safety, improved hashing algorithms, and patched security vulnerabilities. php version 5640 vulnerabilities link
PHP Vulnerabilities: Assessment, Prevention, and Mitigation - Zend Upgrade Roadmap: Moving Beyond 5
A hacked website can lead to data leaks, loss of customer trust, and severe damage to your brand. Upgrade Roadmap: Moving Beyond 5.6.40 allowing attackers to manipulate serialized objects
This application-level vulnerability is common in outdated applications, allowing attackers to manipulate serialized objects, leading to RCE or data corruption.
If you can tell me what CMS (like WordPress) or framework (like Laravel) your site uses, I can provide a more specific checklist for your upgrade process. PHP 5.6: Why you should upgrade - Influential Software