Fatal error: Uncaught Error: trim(): Argument #1 ($string) at \wp-includes\class-wp-query.php

Question

I am using a form to submit table inserted values using <input name=[]> as $_POST to a PHP page. It's a template page of wordpress and I am getting an error.

<?php
/*
Template Name: table page
*/
get_header();
global $wpdb;
$names= $wpdb-> get_results('select stdName FROM wp_nis_student');
?>
<form action="" method="post" >
 <table>
        <thead>
            <tr>
                <th>Student Name</th>
                <th>English</th>
                <th>Maths</th>
            </tr>
        </thead>
        <tbody>

            <?php foreach($names as $name ){?>
                <tr>
                  <td><input type="text" name="name[]" value="<?php echo $name->stdName;>"/</td>
                  <td><input type="text" name="Engmark[]"/></td>
                  <td><input type="text" name="Mathmark[]"/></td>
                </tr>
                <?php }?>
        </tbody>
    </table> 
    <button  type="submit" name="submit" value="Submit">Submit</button>
</form>
<?php
if(isset($_POST['submit'])){
    if(isset($_POST['name'])&& isset($_POST['Engmark']) && isset($_POST['Mathmark'])){
         echo "Names: <br/>";
            foreach ( $_POST["name"] as $name ) 
              echo $name . "<br/>";
            echo "<br/><br/>" .
                 "English : <br/>";
            foreach ( $_POST["Engmark"] as $eng ) 
              echo $eng . "<br/>";

              echo "<br/><br/>" .
                 "Maths: <br/>";
            foreach ( $_POST["Mathmark"] as $mat ) 
              echo $mat . "<br/>";
          }
     else echo "Error: no data";
        }
?>

And I am getting this error because am calling isset($_POST['submit'])

Fatal error: Uncaught Error: trim(): Argument #1 ($string) must be of type string, array given in ..\wp-includes\class-wp-query.php on line 811

Then a made change at the line 811 in class-wp-query.php $qv['name'] = trim( $qv['name'] ); to $qv['name'] = trim( $qv['name[]'] ); it worked fine but i start getting warning on the website

Warning: Undefined array key "name[]" in ..\wp-includes\class-wp-query.php on line 811

How can I resolve it?

Answer 1

The problem is name[], specifically name. When you are choosing what to name your inputs, keep in mind that WordPress itself has inputs and by choosing "name" you're reusing something that's already in use.

E.g. example.com/mypage is converted into index.php?name=mypage behind the scenes, and used to fill up the main query. By reusing name you're putting something in the system that isn't meant to be there. It's expecting a string because page and post names are strings, but your form uses an array.

So rename your name[] input to something else that isn't already reserved. You can look up the WP_Query documentation to see all the options that class takes ( and that you should not use )