When using PHP for web application development, you will often need to use a database. When using a database, error messages are very common. Among them, PHP Fatal error: Call to a member function fetch() is a relatively common error, which occurs when using PDO to query the database. So, what causes this error and how to solve it? This article will explain it in detail for you.

1. Causes of errors

In website development, it is often necessary to retrieve data from the database and process it. PDO is an extension provided by PHP to connect PHP with various databases. When using PDO to operate the database, if the SQL statement is not written correctly or the PDO object is not configured correctly, a Call to a member function fetch() error may occur.

For example, the following code:

$sql = "SELECT * FROM `staff` WHERE `age` > 18";
$res = $pdo->prepare($sql);
$res->execute();
$data = $res->fetch(PDO::FETCH_ASSOC);

If there is an error in the sql query statement, or the PDO object is not configured correctly, the above error may occur in the fetch() method.

2. Error solutions

When an error occurs, it needs to be solved. The following are some common solutions:

1. Confirm whether the SQL statement is correct

SQL statement is the key to access the database. Writing SQL statements correctly is important to avoid errors. When writing SQL statements, you need to pay attention to the following aspects:

(1) It is required to ensure that the keywords, table names, field names, etc. of the SQL statement are in the correct case;
(2) It is required to ensure that the SQL The conditional expression of the statement is correct;
(3) is required to ensure that the syntax of the SQL statement is correct.

2. Confirm whether the database connection is successful

When using PDO, you need to first connect to the database through the PDO::construct() method. If the database connection fails, a Call to a member function will appear. Error in fetch(). Therefore, it is necessary to determine whether the data connection is successful.

try {
    $pdo = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'root', 'password');
} catch (PDOException $e) {
    die ("Error!: " . $e->getMessage() . "<br/>");
}

3. Confirm whether the PDO object is correct

After using PDO to connect to the database, you need to obtain a PDOStatement object. This object represents a prepared statement and can be executed multiple times. If retrieval of the PDO object fails, a Call to a member function fetch() error will occur. When using PDO objects, you need to confirm the following aspects:

(1) Whether the data source and character set used when instantiating the PDO object are correct;
(2) Whether the PDOStatement object exists;
( 3) Whether the PDOStatement object is successfully prepared when executing the query statement.

$sql = "SELECT * FROM `staff` WHERE `age` > 18";
$res = $pdo->prepare($sql);
if(!$res) {
    echo "
PDO::errorInfo():
";
    print_r($pdo->errorInfo());
}
$res->execute();
$data = $res->fetch(PDO::FETCH_ASSOC);

In the above code, when PDO's prepare() method cannot successfully prepare the SQL statement, PDO's errorInfo() method is used to obtain error information.

4. Confirm whether the query has results

When performing a query operation, you need to confirm whether the query statement has results. If the query has no results, a Call to a member function fetch() error will occur.

$sql = "SELECT * FROM `staff` WHERE `age` > 100";
$res = $pdo->prepare($sql);
$res->execute();
$data = $res->fetch(PDO::FETCH_ASSOC);
if(!$data){
    echo "沒有結(jié)果";
    exit;
}

In the above code, when the query result is empty, the program will output "No results" and exit.

3. How to avoid errors

It is more important to avoid errors than to solve them. The following are some common avoidance methods:

1. Error handling mechanism

Adding an error handling mechanism to the program is an important way to avoid errors and can avoid errors. You can add try-catch statements to your code or use the set_exception_handler() method to handle errors.

try {
    $sql = "SELECT * FORM `tableName";
    $res = $pdo->prepare($sql);
    $res->execute();
    $data = $res->fetch(PDO::FETCH_ASSOC);
} catch (PDOException $e) {
    echo $e->getMessage();
    exit;
}

2. Debugging program

Debugging program is a common method to avoid errors. Add debugging statements in the program code to help check the cause of program errors, thereby improving the stability and stability of the program. reliability.

echo $sql;

In the above code, when an error occurs in the SQL statement, $sql can be output to check the correctness of the SQL statement.

Summary

There are many ways to avoid the error of Call to a member function fetch(). Common ones include confirming whether the SQL statement is correct, confirming whether the database connection is successful, confirming whether the PDO object is correct, and confirming Check whether there are results, etc. In daily coding, you should follow correct development specifications, write SQL statements rigorously, and add debugging and error handling mechanisms to avoid errors. In this way, high-quality web applications can be developed.

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