You may do it:
<input type="text" name="your_name" value="">
<?
print $your_name; //will print the name that the user written
?>
If you will write
<input type="text" name="your_name" value="John">
Then when you'll open the page with this input you will always see John first written inside, but you can change it by deleting what's written in the input with a name of your own...