My PHP isset function is not working

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PHP

Pls help!! I am creating a form with php and added isset function to echo whether the value can pass through after clicking the button. However, it only echo not clicked. Can anyone pls help??? I am really desparate on this as my homework deadline is coming.

What I have tried:

<form action="" method="POST">

<table class="tbl-30">

<tr>

<td>Full Name: </td>

<td>

<input type="text" name="full_name" placeholder="Please fill in your full name">

</td>

</tr>

<tr>

<td>Username: </td>

<td>

<input type="text" name="username" placeholder="Please fill in your username">

</td>

</tr>

<tr>

<td>Password: </td>

<td>

<input type="password" name="password" placeholder="Please fill in your password">

</td>

</tr>

<tr>

<td colspan="2">

<input type="submit" name="sumbit" value="Add Admin" class="btn-secondary">

</td>

</tr>

</table>

</form>

<?php

//Submit the value from form and insert into database

//Check whether the button is clicked or not

if(isset($_POST['submit']))

{

// if true, button is clicked

echo "Button Clicked";

}

else

{

// if not true, button is not clicked

echo "Button is not Clicked";

}

?>