<form action="selectDB.php" method="post">
<input type="text" name="studentID" />
<input type="submit" name="send" />
</form>

<?php
if( isset($_POST['send'])){
    $var = $_POST[ 'studentID' ];
    //now you can use the var for an query to your database
    //please note: this very basic, without any security of injection
    $res = mysql_query( 'SELECT `*` FROM `student` WHERE `idStudent` LIKE '%'.$var.'%' ' );
    if( mysql_num_rows($res)){
        $row = mysql_fetch_assoc( $res ); //get one (first) result
    }
}

?>

<script> location.href='editRegistration.php';</script>

<form class="form-horizontal" id="registration" method="post" action="selectDB.php" name="send">
    <input type="text" name="studentID"  align= "right" class="textbox" value="<?php isset( $row[ '0' ] ) ? $row[ 'idStudent' ] : '' ?>" />  <span id="errorID"></span> <br/>
</form>

$res = mysql_query( 'SELECT `*` FROM `student` WHERE `idStudent` LIKE '%'.$var.'%' ' );

$res = mysql_query( "SELECT * FROM `student` WHERE `idStudent` = ".$var);