php if statement from html option value error - php

Question

I am trying to replicate the following:
php if statement html option value
and am following the directions as suggested:
Give name to your select.
<select name="selectedValue">
<option value="Newest">Newest</option>
<option value="Best Sellers">Best Sellers</option>
<option value="Alphabetical">Alphabetical</option>
</select>
In php
Example:
switch($_POST['selectedValue']) {
case 'Newest':
// do Something for Newest
break;
case 'Best Sellers':
// do Something for Best seller
break;
case 'Alphabetical':
// do Something for Alphabetical
break;
default:
// Something went wrong or form has been tampered.
}
However, I am getting the following error:
Notice: Undefined index: selectedValue in C:\xampp\trial.php on line 7
Please help

Answer 1

if you are sending form data then you have to set method post in your form tag or if you are using ajax send it by type post and in your php file put code inside if condition after form has been posted like isset($_POST['selectedValue'])
<form method="post" action="url to your php file">
<select name="selectedValue">
<option value="Newest">Newest</option>
<option value="Best Sellers">Best Sellers</option>
<option value="Alphabetical">Alphabetical</option>
</select>
<button type="submit">Submit</button>
</form>

Answer 2

That's happens because you don't always have the key selectedValue in your POST array.
if(isset($_POST['selectedValue'])){
// your code
}

Answer 3

My problem was solved after adding a submit button within the form tag of HTML
<form action="#" method="post">
<select name="selectedValue">
<option value="Newest">Newest</option>
<option value="Best Sellers">Best Sellers</option>
<option value="Alphabetical">Alphabetical</option
</select>
<input type="submit" style="float:left; margin-left: 2%;" name="submit" value="Search"/>
</form>