PHP Cannot redeclare function, previously defined?

Question

I'm very new to PHP and I've come across an issue I can't solve. I am trying to create a page that adds a new row to a database. I keep getting an error saying I cannot redeclare the checkIfAdminExists function that's defined in currentSessionData.php (prob gonna rename)

I suspect the issue has something to do with the fact that I'm setting the header location to the current page (create_admin.php) to process form data through POST, and then when the page is loading it's trying to redeclare the function. I've tried including the file inside

if (!function_exists('checkIfAdminExists')),

but I still get the same error. What am I doing wrong? Is there a better way to approach handling form data with a function?

create_admin.php

<?php
require_once('currentSessionData.php');

if (isset($_POST['newUsername']) && isset($_POST['newPassword'])) {
    if (checkIfAdminExists($_POST['username'], $_POST['password'])) {
        // admin account already exists
        echo '<script language="javascript">';
        echo 'alert("This admin account already exists");';
        echo '</script>';
        header('Location: create_admin.php');
    }
    else {
        //create new admin account in database
        $username = $_POST['newUsername'];
        $password = $_POST['newPassword'];

        $username = mysql_real_escape_string($username);
        $username = mysql_real_escape_string($password);
        $sqlQuery = "INSERT INTO table_test (username, password)
        VALUES ('$username', '$password')";

    }
}
?>

currentSessionData.php

<?php

function checkIfAdminExists($username, $password) {
require_once("db_connection.php");
$sql = "SELECT personid, username, password FROM table_test";
$result = $dbcon->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        if ($row["username"] == $username && $row["password"] == 
$password) {
            return true;
        }
    }
}
else {
    return false;
}
$dbcon->close();
}

?>

Answer 1

As per your comment, you are placing the function_exists check in the wrong place, change your code to the following:

if (!function_exists('checkIfAdminExists')) 
{
    function checkIfAdminExists($username, $password) {
        require_once("db_connection.php");

        $sql = "SELECT personid, username, password FROM table_test";
        $result = $dbcon->query($sql);

        if ($result->num_rows > 0) {
            while($row = $result->fetch_assoc()) {
                if ($row["username"] == $username && $row["password"] == $password) {
                    return true;
                }
            }
        } else {
            return false;
        }

        $dbcon->close();
    }
}

Now, with the above code, the function will only be defined if it doesn't already exist.

Note: Please stop using mysql_* as it has been officially deprecated and removed in PHP 7. It would be wise to start learning mysqli_* or PDO and to make use of prepared statements.

Update #1

You are setting the username variable twice:

$username = mysql_real_escape_string($username);
$username = mysql_real_escape_string($password);

It should be:

$username = mysql_real_escape_string($username);
$password = mysql_real_escape_string($password);

Update #2

As per the documentation of mysql_real_escape_string, it takes an optional second parameter:

The MySQL connection. If the link identifier is not specified, the last link opened by mysql_connect() is assumed. If no such link is found, it will try to create one as if mysql_connect() had been called with no arguments. If no connection is found or established, an E_WARNING level error is generated.

With the above in mind, it means that your connection has not been established. So the next logical question is: Where is your connection being established in create_admin.php?