if change input value from inspect element

hello everyone i have a problem

if(isset($_POST['submit'])) {$cars = $_POST['cars']; echo $cars;}

<form>
<input type="checkbox" name="cars[]"  value="1" />
<input type="checkbox" name="cars[]"  value="2" />
<input type="checkbox" name="cars[]"  value="3" />
<input type="checkbox" name="cars[]"  value="4" />
<button>add</button>
</form>

how can i stop changeing value if user change the input value from inspect element , or if user change the value alert me

i try this

$allowe = ['1','2','3','4'];
if(in_array($_POST['cars'],$allowe)) {echo $_POST['cars'];}

but is not working is only work for one value