Description:
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It seams that whenever PDO::MYSQL_ATTR_SSL_CERT is set to something, PHP will try to validate the database certificate, regardless of the PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT value.

Test script:
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I've successfully used the following value combination when connecting to MySQL servers that uses a certificate signed by a trusted Certificate Authority (CA).

PDO::MYSQL_ATTR_SSL_CAPATH=/etc/ssl/certs
PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT=true

Based on PHP documentation (https://www.php.net/manual/en/ref.pdo-mysql.php), I thought I could simply toggle MYSQL_ATTR_SSL_VERIFY_SERVER_CERT to false to allow connections to database servers that use self-signed certificate (i.e. for which the CA certificate is absent from /etc/ssl/certs). 

PDO::MYSQL_ATTR_SSL_CAPATH=/etc/ssl/certs
PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT=false



Expected result:
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PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT set to false would prevent PHP from validating the database certificate regardless of the MYSQL_ATTR_SSL_CAPATH value.


Actual result:
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Doing so constantly leads to the following error: 

PDO::__construct(): SSL operation failed with code 1. OpenSSL Error messages: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed

Surpringly, the following combinations works:

PDO::MYSQL_ATTR_SSL_CAPATH=null
PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT=[false|true]

In which case I believe the database server switches to unencrypted protocol, if it allows it, which is not what we want.