Why am I getting the \'Call to a member function query() on Null\' Error in PHP?

Troubleshooting "Call to a Member Function query() on Null" Error

In the code provided, the error "Fatal error: Call to a member function query() on null" occurs in line 7 of login.php. This error indicates that the $db variable, which is expected to be a mysqli object, is null when attempting to execute a query using the query() method.

Resolution:

Upon closer inspection of the code, we notice that the initialization of the $db variable is placed outside the user_exists() function, where it is used. This means that within the function, $db is not defined and is therefore null. To resolve this issue, we need to pass $db as a parameter to the user_exists() function. Here's the corrected code:

<code class="php">function user_exists($db, $username) {
    // ...
}</code>

And in login.php, you can now call the function like this:

<code class="php">$result = $db->query("SELECT COUNT(UserId) FROM users WHERE UserName = '$username'");</code>

By passing $db as a parameter, we ensure that the function has access to the mysqli object and can successfully execute the query.

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