Here is a very basic example of hoe to do it:
<?php
// Do your usual mysql connect routine
$query = "SELECT accountNumber FROM accounts ";
$result = mysql_query($query);
while ($row = mysql_fetch_array($result)) {
$accountNumber = $row['accountNumber'];
}
?>
<form action="" method="post">
<input type="text" name="accountNumber" value="<?php print $accountNumber;?>">
<input type="submit" name="submit" value="Process">
</form>