How to look for a constant value by posting the column name in PHP mysql?

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PHP

MySQL

I have created this code to retrieve multiple user data where the pincode = the pin-code i am posting through post method and $value = 't'($value = a column name(i want to send the column name using post method(column name= a, b, ho, ll, c, d)) where i want it to look for 't'). my code is not complete and i don't know how to do it can anyone help me with this? Later i want it to connect with android app and list view the retrieved data.

What I have tried:

My function in DbOperations.php

public function gettestuser($pin){
        $stmt = $this->con->prepare("SELECT * FROM test_category WHERE $value = 't' pin = ?");
        $stmt->bind_param("s",$pin);
        $stmt->execute();
        return $stmt->get_result()->fetch_assoc();
    }

My gettestuser.php

<?php
require_once '../include/DbOperations.php';
 
$response = array();
 
if($_SERVER['REQUEST_METHOD']=='POST'){
if(isset($_POST['reg_pin'])){
 
    $db = new DbOperations();
 
    $test_category = $db->gettestuser($_POST['reg_pin']);
 
    var_dump($test_category);
 
        $response['error'] = false;
        $response['pid'] = $test_category['pid'];
        $response['name'] = $test_category['name'];
        $response['pin'] = $test_category['pin'];
        $response['a'] = $test_category['a'];
        $response['b'] = $test_category['b'];
        $response['ho'] = $test_category['ho'];
        $response['ll'] = $test_category['ll'];
        $response['c'] = $test_category['c'];
        $response['d'] = $test_category['d'];
 

 
}else{
    $response['error'] = true;
    $response['message'] = "Required fields are missing";
}
}
 
echo json_encode($response);
?>