php what does checkbox default value 'on'

Question

Recently while working with checkbox, I've to go through following test code:

<?php if ( isset( $_POST['myname'] ) ) {
$myvalue = $_POST['myname'];
echo $myvalue; } ?>

<form method="post">
<input type="checkbox" name="myname" />
<input type="submit" name="send" /></form>

So, what output I got after form submission is when checkbox is checked it prints 'on' else nothing (if value is not provided on checkbox). And isn't it supposed to print 1(true) on checking the checkbox. What that 'on' means in PHP?

Answer 1

If you don't provide value attribute for checkbox, it's value will be on

With

<input type="checkbox" name="myname" />

$_POST['myname'] value is on

With

<input type="checkbox" name="myname" value="1" />

$_POST['myname'] value is 1

In both cases when checkbox is unchecked , it doesn't present in $_POST (not set)