PHP Forms

I have the following php code to asks for a new student to fill out a registration form. Using php, take what the user enters and put it into the "students" table. But when i select * from students in mysql, it didn't show, why? <HTML> <BODY bgColor=#ffffff> <?php if ($submit) { // process form $db = mysql_connect ("localhost", "root", "mollier"); $select=mysql_select_db ("mydb"); $sql = "INSERT INTO students (student_id, firstname,lastname, address) VALUES ('$student_id', '$firstname', '$lastname', '$address')"; $result = mysql_query($sql); echo "Thank you! Information entered.\n"; } else{ // display form ?> <FORM onsubmit="return formvalidation(this)" action="<?php echo $PHP_SELF?>" method=post><INPUT type=hidden value="New Student Registration - BBS" name=FORM_NAME> <P style="LINE-HEIGHT: 100%; BACKGROUND-COLOR: #0000ff" align=center><B><FONT color=#ffffff size=5><BR>REGISTRATION FORM<BR><BR></FONT></B></P> <PRE><B> <FONT color=#ff0000>REQUIRED QUESTIONS:</FONT></B> <BR> 1. What is your student id number?&nbsp;<INPUT tabIndex=2 maxLength=46 size=46 name=student_id><BR> 2. What is your name?<BR> First name:<input type="Text" name="firstname"><br> Last name:<input type="Text" name="lastname"><br> Address: <input type="Text" name="address" size="60"><BR> </PRE> <UL> <LI> <P style="BACKGROUND-COLOR: #ffffff"><font color="#FF3300">Please click the Submit Button only once</font> </P> <LI> <P style="BACKGROUND-COLOR: #ffffff"><font color="#FF3300">Then wait for your confirmation screen</font> </P> <LI> <P style="BACKGROUND-COLOR: #ffffff"><font color="#FF3300">Save the confirmation screen because it has information you will need</font> </P> </LI></UL> <P style="BACKGROUND-COLOR: #ffffff"><INPUT type=submit value="Submit Registration">&nbsp;</P> <P style="BACKGROUND-COLOR: #ff0000" align=center>End of Form-Thank You!</P></FORM> <HR> <H5 style="BACKGROUND-COLOR: #ffffff"> </H5> <?php } // end if ?> </BODY></HTML>