Checkbox arrays php

Hi

Im using the following code to create checkboxes in a form. What I need, is the boxes the user checked, to remain checked after the form has been submitted. The reason for this is so that the user will not have to re-check the boxes if they would like to submit the form again. How can I do this using php code ?


if ($db_found) {

	$sql="select cname, cid from course";

	//query the database
	$rs=mysql_query($sql) or die($sql.">>".mysql_error());

	//count how many records found
	$num=mysql_num_rows($rs);

if($num>0){ //check if
<FORM NAME ="form1" METHOD ="POST" ACTION ="student_reg.php">
//create array loop
	while($row=mysql_fetch_array($rs)){

		// extract row and echo the array
		extract($row);

	echo "$cname"."<Input type = 'Checkbox' Name ='chk1[]' value ='$cid'>";

}

	<INPUT TYPE = "Submit" Name = "Submit1"  VALUE = "Register">


	</FORM>
}

}

if $cid is in array $_POST[‘chk1’], add ‘checked’ to the echo.