English
English
In PHP, mysqli_stmt::attr_get is a function used to obtain the value of the prepared statement attribute. Although this function is not common, it may be used to read the internal state or configuration information of the statement in a specific application scenario, such as performance tuning or diagnosing the database communication state of the intermediate layer. In multi-connection environments (such as handling multiple database connections at the same time or using a connection pool), it is particularly important to use this function correctly. Below we will analyze several key issues that need to be paid attention to when using mysqli_stmt::attr_get from multiple perspectives.
In mysqli architecture, a mysqli_stmt object is bound to a specific mysqli connection. When you create a new connection:
$conn = new mysqli("localhost", "user", "password", "database");
$stmt = $conn->prepare("SELECT * FROM users WHERE id = ?");
At this time $stmt explicitly depends on $conn . This means that you cannot reuse the $stmt on another connection, nor can you use the $stmt attribute for the statement generated by another connection. This is especially important in multiple connections (such as multiple different hosts or read-write separated master-slave databases) and can easily lead to logical errors or null value exceptions.
PHP itself is run single thread, but in FPM mode or asynchronous frameworks, multiple requests may operate different connections simultaneously. Although the mysqli extension is thread-safe (provided that thread-safe support is enabled during PHP compilation), mysqli_stmt::attr_get does not have cross-connection state visibility. If you call attr_get on a connection, the read results only represent the connection context and do not have global reference significance.
The attribute keys supported by the attr_get() method are very limited, and the behavior of these attributes may vary across versions of MySQL or MariaDB. For example, if you use the following code:
$value = $stmt->attr_get(MYSQLI_STMT_ATTR_CURSOR_TYPE);
You expect to read the cursor type (such as MYSQLI_CURSOR_TYPE_READ_ONLY ), but if the underlying driver or version does not support the property, the return value may be false and trigger a warning. This requires special attention in multi-connection, multi-version database clusters, and all possible database behaviors must be tested.
After the connection fails or is closed actively, even if the $stmt variable still exists, its underlying handle is invalid. Calling attr_get() at this time will return false and an error may be reported. For example:
$conn->close();
$value = $stmt->attr_get(MYSQLI_STMT_ATTR_CURSOR_TYPE); // fail
In high concurrency environments, such problems can be more hidden if the connection is temporarily recycled (such as the "rental-return" mechanism in the connection pool). Therefore, before calling attr_get , you should make sure that the connection is still available.
Although attr_get() is a read-only function, the read of certain attribute values may cause implicit behavior, such as initializing some client buffering mechanisms. This behavior may not be clear in the official documentation, but may occur in specific implementations. Therefore, it is recommended to limit the use of attr_get() to diagnostic code to avoid frequent calls in the main business logic.
Here is an example using mysqli_stmt::attr_get for reading cursor type:
