Can network issues between the PHP server and the MySQL server cause 'mysqli_query' errors? How can I diagnose them?

Yes, network issues (firewall restrictions, DNS resolution problems, server downtime) can definitely cause `mysqli_query` errors. Diagnose these by first verifying that the MySQL server is running and accessible from the PHP server (e.g., using `ping` or `telnet`). Check firewall rules to ensure that traffic on the MySQL port (typically 3306) is allowed. Also, verify DNS resolution is working correctly if you're using a hostname to connect to the database.