PHP Works but gives sql SYNTAX error

Question

I just wrote this bit of code which echo's out what it's supposed to but after the echo statement it give me the error-

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1

why's this happening? how do i fix it?

<?php
$myclasses = explode(',', $_SESSION['classlist']);  
$theirclasses = explode(',', $user_info['classlist']);
$common_classes = array_intersect($myclasses, $theirclasses);

if (count($common_classes) > 0) {     

    foreach ($common_classes as $class) {
        $classes = mysql_query("SELECT * FROM classes WHERE class_id = ".$class) or die(mysql_error());
        while($currentRow = mysql_fetch_array($classes)){
        echo $currentRow['class_name'];
        }
    }  
}
else {
}

?>

Answer 1

Try wrapping your query with quote:

$classes = mysql_query("SELECT * FROM classes WHERE class_id = '".$class."'") or die(mysql_error());

or change your query altogether by using PDO. Because, mysql_* function are deprecated.