How do I solve the warning undefined array key

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PHP

WordPress

In a WordPress project, I am inserting a PHP file to do some specific form code.
I wanted to change the form from a drop-down list to a group of checkboxes, and now I am having thousands of warning: undefined array or undefined variable:

function calculdebit()
{
	global $wpdb;
	$plateauType =  $_POST['plateauType'];

....
}

<div id="select-2" class="champ_outil">
					<label for="select-2-label">
                    Quel est le plateau type ?</label>
                    <select id="plateauType" name="plateauType[]">
							<!-- <option value="1">
                    </option> -->
							<option id="PLATEAU-1" value="2"  
                    <?= (isset($_POST['plateauType']) && 
                    $_POST['plateauType'] === '2') ? 'selected' : ''; ? 
                    >> plateau-1 </option>
							<option id="PLATEAU-2" value="3" 
                            <?= (isset($_POST['plateauType']) && 
                            $_POST['plateauType'] === '3') ? 
                     'selected' : ''; ?>> plateau-2 </option>
							<option id="PLATEAU-3" value="4" <?= 
                      (isset($_POST['plateauType']) && 
                      $_POST['plateauType'] === '4') ? 'selected' : ''; ? 
                      >> plateau-3 </option>
							<option id="PLATEAU-4" value="5" <?= 
                      (isset($_POST['plateauType']) && 
                      $_POST['plateauType'] === '5') ? 'selected' : ''; ? 
                      >> plateau-4 </option>
							<option id="PLATEAU-5" value="6"  <?= 
                      (isset($_POST['plateauType']) && 
                       $_POST['plateauType'] === '6') ? 'selected' : ''; ? 
                       >> plateau-5 </option>
						</select>
            <pre> <?
            
            echo $plateauType[1]; ?> 

//trying to see if there is something in the variable but it gives this error too Warning: Trying to access array offset on value of type null.

What I have tried:

I've tried to add " @global var $name_of_variable" on the top of code it does not work, and I also tried to name the variable the same name in PHP and in HTML code.

Any help would be so much appreciated, because I am stuck for a week.