'How to keep value of selected value after form submission?

I know there are a lot of questions related to it but believe me, no solution is working for me. 

 <form action="index" method="post" >
            <select id="s" name="dtype" class="dropdown" required
                    style="float: left; text-align: left;width: 20%; margin: 10px;">
                <option value="400">Select Data Type</option>
                <option value="401">Current</option>
                <option value="402">Voltage</option>
                <option value="403">kWh</option>
            </select>
</form>

What I have tried

I have tried the following 

<option <?php if ($_GET['dtype'] == '401') { ?>selected="true" <?php }; ?>value="401">Current</option>
<option <?php if ($_GET['dtype'] == '402') { ?>selected="true" <?php }; ?>value="402">Voltage</option>
<option <?php if ($_GET['dtype'] == '402') { ?>selected="true" <?php }; ?>value="402">kwh</option>

Tried below solution

  1. Keep values selected after form submission
  2. PHP keep dropdown value after submit

All the solution gives me unidentified index: dtype error

Any help would be highly appreciated.

phphtmlformsyii2-advanced-app


Solution 1:[1]

In your form you have: method="post" And then you use the $_GET['dtype'] Either change the method of the form to method="get" or change the if conditions to check for $_POST['dtype'].

That's why it says unidentified index. You are sending the data as post and trying to get it from the $_GET array.

Apart from this, if you are using xhtml the attribute value should be selected="selected" whereas if you use an html 5 doctype it should be just selected.

Solution 2:[2]

The first option (Select Data Type) must have an empty value in order for "required" option to work.

<form action="index.php" method="POST">
    <select id="s" name="dtype" class="dropdown" required style="...">
        <option value="">Select Data Type</option>
        <option value="401" <?php if(isset($_POST['dtype']) && $_POST['dtype']=="401") {echo'selected';} ?>>Current</option>
        <option value="402" <?php if(isset($_POST['dtype']) && $_POST['dtype']=="402") {echo'selected';} ?>>Voltage</option>
        <option value="403" <?php if(isset($_POST['dtype']) && $_POST['dtype']=="403") {echo'selected';} ?>>kWh</option>
    </select>
</form>

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2 George

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