This critical vulnerability occurs in mbstring regular expression functions when they are supplied with invalid multibyte data. It can allow a remote attacker to compromise the target system.
A heap-based buffer over-read in the PHAR extension may allow attackers to read memory past actual data while parsing filenames. php version 5640 vulnerabilities link
PHP version 5.6.40 was released on January 10, 2019, as the final security release for the PHP 5.6 branch. While it addressed several critical security bugs at the time, it reached its official , meaning it has not received official security updates or bug fixes for over seven years. Key Vulnerabilities in PHP 5.6.40 it reached its official