Can I send $_POST[] from the onchange of the form to a javascript function like this
<select name="slct" onchange="rohan('measurement_conversion', <?php isset($_POST["slct"])?$_POST["slct"]:"nothing" ?>)">
If no then How can I do this ?
It looks like you're basically trying to set a JavaScript variable by printing it out based on some PHP conditions. You can totally do this, but it looks like you want to pass a string through so you'll need to wrap it with quotes.
You should also do something to sanitize that value, it's dangerous to let someone potentially change your HTML by just putting something in the URL.
<select name="slct" onchange="rohan('measurement_conversion', '<?php isset($_POST["slct"])?$_POST["slct"]:"nothing" ?>')">
You could submit the form and cause a postback with javascript in the onchange event: