I insert about 1.1 Mb of Data into a Mysql Field of type LONGBLOB. This is far away from the maximum supported length of a LONGBLOB field.
The insert seems to work.
It was the MYSQL_ATTR_MAX_BUFFER_SIZE which is 1MB by default.
This fixed the issue:
$pdo->setAttribute(PDO::MYSQL_ATTR_MAX_BUFFER_SIZE, 1024*1024*50); // 50 MB