<form action="update.php" method="post">

<input type="text" name="userdate" value="<?= $b ?>" /> 

<input type="submit" name="subjoin" value="Update">

</form>

and update.php

$c= $_POST["$b"];
echo "value".$c; //its not working

<?php
$b="my value!";
?>
<form action="update.php" method="post">
<input type="text" name="userdate" value="<?php echo $b ?>" /> 
<input type="submit" name="subjoin" value="Update">
</form>

and update.php

if(!empty($_POST["userdate"]))
{
$c= $_POST["userdate"];
echo "value".$c; //its not working
}