how to call function form javascript inside php?

You can't.

PHP runs on the server side before the page is downloaded by the client. JavaScript runs on the client side after the page has been downloaded by the client.

Therefore it is impossible for PHP to call a JavaScript function. It is possible to write JavaScript with PHP as example your code shows but not to execute it.

What you're doing would work except that you're using single quotes inside single quotes. Change it to:

document.getElementById("msg").innerHTML = "asdsd";

It's a different matter, of course, get a return value back from JavaScript to the PHP script, as the PHP script has finished running before the JavaScript one starts.

Writing JavaScript in PHP is nothing but writing JavaScript for the DOM. You can write it in PHP. use within PHP or directly.

Example:

<?php
echo "
    <script type=\"text/javascript\">
        function greeting(name){
            alert(name);
        }
    </script>
";
?>
<script type="text/javascript">
greeting('srihari');
</script>

I hope that this will help you.

And don't use single quotes within single quotes,

You can simply call that function and this will run after PHP code will send to your browser but it will give you result as you want

You can use this

<?php
ob_start();
require 'connect.inc.php';

session_start();
$usrequest = $_REQUEST['usr'];
$pwdrequest = $_REQUEST['pwd'];

$id = 'msg';

echo '<script type="text/javascript" language = "javascript">

    function send ()
    {
        document.getElementById('msg').innerHTML = "asdsd";
    }
echo '<script type="text/javascript>send();</script>"'
</script>';    
?>

There exist a couple of attempts to make Google's V8 javascript engine available from within PHP, e.g. https://github.com/preillyme/v8js