Why can't mysqli_stmt::attr_get get the attribute value?

When using PHP's MySQLi extension, the mysqli_stmt::attr_get function is used to get the attribute value of the preprocessed statement object. However, many developers will encounter the problem that they cannot correctly obtain the property value when calling this function. This article will analyze the possible causes of this phenomenon in depth and provide corresponding solutions.

1. The role and usage scenarios of mysqli_stmt::attr_get

The mysqli_stmt::attr_get method is used to obtain the specified preprocessing statement attributes, which are usually set by mysqli_stmt::attr_set . Common properties include:

• MYSQLI_STMT_ATTR_UPDATE_MAX_LENGTH

• MYSQLI_STMT_ATTR_CURSOR_TYPE

• MYSQLI_STMT_ATTR_PREFETCH_ROWS

Let's give a simple example:

 $mysqli = new mysqli('m66.net', 'user', 'password', 'database');

$stmt = $mysqli->prepare("SELECT * FROM users WHERE id = ?");
$stmt->attr_set(MYSQLI_STMT_ATTR_CURSOR_TYPE, MYSQLI_CURSOR_TYPE_READ_ONLY);

$cursorType = $stmt->attr_get(MYSQLI_STMT_ATTR_CURSOR_TYPE);
echo "Cursor Type: " . $cursorType;

If everything works fine, $cursorType should return the cursor type value set.

2. Why can't the attribute value be obtained?

1. The attribute is not set correctly
   attr_get can only obtain attribute values ​​that have been set through attr_set . If the attribute is never set, attr_get returns false or unexpected value.

2. Some properties only support writing but not reading <br> Some properties are written only, and attr_get cannot read their values, resulting in invalid returns.

3. MySQLi driver version restrictions or bugs
   Different versions of PHP and MySQLi drivers support for attributes is not exactly the same, and some versions of attr_get may have implementation defects.

4. Incorrect attribute constant or type <br> If the attribute constant passed in during call is not a valid MYSQLI_STMT_ATTR_* , it will not be read.

3. Solution

1. Make sure the properties are set <br> Before calling attr_get , make sure that the corresponding attribute has been correctly set with attr_set . For example:

 $stmt->attr_set(MYSQLI_STMT_ATTR_PREFETCH_ROWS, 10);
$value = $stmt->attr_get(MYSQLI_STMT_ATTR_PREFETCH_ROWS);

2. Check PHP and MySQLi versions <br> Make sure that the PHP version and MySQLi extensions you are using are up to date, or at least support the properties you want to use. Upgrading PHP versions usually solves compatibility issues.

3. Avoid reading and writing-only attributes <br> Check the official documentation to confirm whether the target attribute supports reading and avoid reading and writing only attributes.

4. Use error handling and debugging information <br> After calling attr_get , check the return value and use mysqli_stmt_error to capture the error message to help locate the problem:

 $value = $stmt->attr_get(MYSQLI_STMT_ATTR_CURSOR_TYPE);
if ($value === false) {
    echo "Failed to get attribute: " . $stmt->error;
}

5. Use alternatives <br> If you need to confirm the status of certain attributes, you can consider using program logic or querying database server parameters instead of directly reading the attributes.

4. Summary

Mysqli_stmt::attr_get is limited in use by whether the attribute has been set, the PHP/MySQLi version and the read and write characteristics of the attribute itself. When the property value cannot be obtained, first make sure that the property has been set and the version used supports reading the property. In addition, reasonable error handling and version upgrade are also the key to troubleshooting and solving problems.