Users running versions prior to 5.6.40 are affected by several critical vulnerabilities that this specific release was designed to patch:
Since 5.6.40 is the last scheduled release, it remains vulnerable to newer threats discovered after 2019, such as: php version 5640 vulnerabilities link
Because this version is End-of-Life (EOL), any vulnerabilities discovered after its final release remain unpatched by the official PHP development team. Core Vulnerabilities in PHP 5.6.40 Users running versions prior to 5
A flaw in the xmlrpc_decode function that can lead to information disclosure or crashes. php version 5640 vulnerabilities link