There is no single “master link” labeled "5640." Instead, you must look at the aggregate of Common Vulnerabilities and Exposures (CVEs) that affect version 5.6.40.
You want a link to a list of flaws. But the real risk is not the list; it is the lack of a fix. Here is why collecting CVEs for 5.6.40 is a losing battle: php version 5640 vulnerabilities link
Some notable CVEs that affect 5.6.40:
PHP 5.6.40 is a relatively old version of PHP, and while it's known that older versions may have vulnerabilities that have been discovered and patched in later versions, specific vulnerabilities can include: There is no single “master link” labeled "5640
PHP 5.6.40 should NOT be used in production - it has many known, unpatched vulnerabilities. Upgrade to PHP 7.4+ (or PHP 8.x) immediately for security. Here is why collecting CVEs for 5
Important Note: There is no official PHP version "5.6.40" in the standard PHP release history. The official versions were 5.6.39 and then 5.6.40 (Release Date: Jan 10, 2019). However, given the high likelihood of a typo, this post covers PHP 5.6.40 (the last official security release of the 5.6 branch) and also addresses the possibility you meant the 5.6.4.0 alpha build or a general search for CVE links.