display a form just in a particular case using control structure - php

Question

I'm a beginner in PHP, I am working on a carpooling website and I want to display a form in the user account only if he didn't enter his car yet here is my code and unfortunately it didn't work and I haven't got any error on my screen. Please help me if you can !!
<div id="edit-settings" class="tab-pane">
<?php $user_id=$_SESSION['user']['id'];
$res = $db->query('SELECT COUNT(*) FROM car where $user_id');
$get_total = $res->fetchColumn();
if($get_total==0){
echo 'please enter a car by filling the form below'; ?>
<form action="car.php" method="post">
<br />
<label class="control-label" for="inputEmail">
Brand *
</label>
<input class="span4" type="text" placeholder="Brand *" name="brand" maxlength="64" required="">
<br />
<br />
<label class="control-label" for="inputEmail">
Model *
</label>
<input class="span4" type="text" placeholder="Model *" name="model" maxlength="64" required="">
<br /><br />
<label class="control-label" for="inputEmail">
Confort *
</label>
<select id="lst_confort" class="span4" name="confort">
<option value="Basique">Basic</option>
<option value="Normal">Normal</option>
<option value="Confortable">Confortable</option>
<option value="Luxe">Luxary</option>
</select>
<br /><br />
<label class="control-label" for="inputEmail">
Color *
</label>
<select id="lst_confort" class="span4" name="couleur">
<option value="0" selected> choose a color</option>
<option value="0"> white</option>
<option value="1">black</option>
<option value="2">blue</option>
<option value="3"> Red</option>
<option value="4"> Orange </option>
<option value="5"> Green</option>
<option value="6"> Yellow </option>
</select>
<br /><br />
<label class="control-label" for="inputEmail">
Nb places *
</label>
<select name="places" id="lst_NbPlaces" class="span2" required="">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
</select>
<br />
<br />
<button type="submit" class="btn btn-success">
Submit</button>
<br />
<br />
<br />
<label class="text-info">
* required field
</label>
</form>
<?php } else {
echo "You have already entered car !";
}
?>

Answer 1

Your query is wrong:
SELECT COUNT(*) FROM car where $user_id
This is in single quotes, so it is interpreted as exactly what you see above; $user_id is seen as literal text, not a variable. That is not a valid query. I suspect you mean something like
$res = $db->query('SELECT COUNT(*) FROM car where user_id = ' . $user_id);
(Change user_id to match your column name as needed.)
Note that this is extremely vulnerable to SQL injection and is therefore entirely unsafe for real-world use.
Also, in the future, you can try checking the database errors and logs to determine what went wrong.

Answer 2

Your query has a syntax error:
$res = $db->query('SELECT COUNT(*) FROM car where $user_id');
^---
You have no comparison, so in effect you're doing
SELECT COUNT(*) ... WHERE true
and are counting ALL records in the table. As soon as you had one user enter a car, ALL users were treated as having cars. Perhaps you want something more like
$res = $db->query('SELECT COUNT(*) FROM car where user = $user_id');
^^^^^^^