Warning message about undefined $_POST array key in MySQL query

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1

I’m implementing a multiple dropdown menu that retrieves data from a MySQL database using PHP and JavaScript. The issue arises when the page first loads, as I encounter an error indicating that the $_POST array is undefined, which is essential for executing my MySQL query and displaying the related chart.

The error message I’m seeing is: “Warning: Undefined array key ‘titulos’ …”. After my first form submission, everything works correctly.

How can I establish a default value for this $_POST variable to prevent the error? Here’s part of my form:

<form name="myform" action="" method="post" style="text-align:center;margin-top:2%;">
    <select name="magazines" style="font-size:16px;" onclick="getSeries(this.value)">
        <option value="">Choose a Magazine</option>
        <?php
            $magazines = getMagazines();
            foreach($magazines as $magazine){
                echo '<option value="'.$magazine['magazine_code'].'">'.$magazine['Magazine_name'].'</option>';
            }
        ?>
    </select>
    <select name="titulos" style="font-size:16px;">
        <option value="">Choose a Series</option>
    </select>
    <input type="submit" value="Send" style="font-size:16px;">
    
    <?php
    $series_tit = $_POST['titulos'];
    $query = "SELECT * FROM chartdata WHERE Serie_name = '$series_tit'";
    ?>
</form>

The PHP functions I’m using are:

function getMagazines() {
    $conn = databaseConnection();
    $res = $conn->query("SELECT * FROM categorias");
    while ($row = $res->fetch_assoc()) {
        $data[] = $row;
    }
    return $data;
}

if (isset($_GET["magazine_code" ])) {
    echo getSeries($_GET["magazine_code"]);
}

function getSeries($magazine_code) {
    $conn = databaseConnection();
    if (!$conn) {
        return false;
    }
    $magazine_code = htmlspecialchars($magazine_code);
    $res = $conn->query("SELECT * FROM chartdata WHERE magazine_code ='$magazine_code'");
    while ($row = $res->fetch_assoc()) {
        $data[] = $row;
    }
    return json_encode($data);
}

Here’s how my JavaScript function looks:

function getSeries(magazineCode) {
    const seriesDropDown = document.forms["myform"].titulos;

    if (magazineCode.trim() === "") {
        seriesDropDown.disabled = true;
        seriesDropDown.selectedIndex = 0;
        return false;
    }

    fetch(`functions_sales.php?magazine_code=${magazineCode}`)
    .then(response => response.json())
    .then(function(series) {
        let options = '<option value="">Choose a Series</option>';
        for (let sery of series) {
            options += `<option value="${sery['Serie_name']}">${sery['Serie_name']}</option>`;
        }
        seriesDropDown.innerHTML = options;
        seriesDropDown.disabled = false;
    });
}

I’ve attempted to set a default selected option in the dropdown, but I’m still facing the same warning when the page loads.

2

just wrap your php code in an if statement to check if the post exists first. something like if(isset($_POST['titulos']) && !empty($_POST['titulos'])) { $series_tit = $_POST['titulos']; // your query here } - this way it won’t try to access the post variable before form submission.