致命错误：未捕获错误：调用未定义函数mysql_close（）

I'm learning about MySQL prepare statement and when I run the following code, I get the PHP Fatal error: Uncaught Error: Call to undefined function mysql_close(). The output is correct and I just can't figure out why such an error occurs. Thanks for your time!

$conn3 = new mysqli(DB_HOST,DB_USERNAME,DB_PASSWORD,DB_NAME);

$event_type = $jarray['event_type'];
$key = "submission";

$search = $conn3->prepare("SELECT * FROM metadb USE INDEX(field_id) WHERE event_type = ? AND event_field = ?");
$search->bind_param('ss', $event_type, $key); 
$search->execute();
$search->store_result();

echo $search->num_rows;

mysql_close($conn3);

Use $conn3->close(); instead of mysql_close($conn3);

You are using mysqli to open connection and then trying to close connection using mysql.

Suggestion : You are mixing mysqliobject oriented and procedural in your code I will suggest you to stick with one(preferably Object Oriented). I will also suggest you to start using PDO instead of mysqli. reason explained here

For more detailed information about mysqli you can refer this documentation.

For more detailed information about PDO you can refer this documentation.