if ( isset ($_POST['meter']) )

<form method="post">
<input type="text" name="meter" value="<?php echo $meter ?>"> meter<br>
<input type="text" name="yard" value="<?php echo $yard ?>"> yard
<input type="submit">
I have the code above,
If I enter "meterNumber" in the input box "meter" and click the submit button,
"yardNumber" will be seen the input box "yard".

I like to make it like the following.
If I enter "meterNumber" in the input box "meter",
"yardNumber" will be simultaneously seen the input box "yard" without clicking the submit button.