This is a classic question, with two solutions. If you want to remove exactly one comma, which may or may not be there, use:

if (substr($string, -1, 1) == ',')
{
  $string = substr($string, 0, -1);
}

If you want to remove all commas from the end of a line use the simpler:

$string = rtrim($string, ',');

The rtrim function (and corresponding ltrim for left trim) is very useful as you can specify a range of characters to remove, i.e. to remove commas and trailing whitespace you would write:

$string = rtrim($string, ", \t\n");

Try:

$string = "'name', 'name2', 'name3',";
$string = rtrim($string,',');

Better way is to use implode(). Example:

$array = array("top" => array("one" => "inner one", "two" => "inner two"));
foreach($array as $key=>$val){
    $form = array();
    $sql = "INSERT INTO $key SET ";
    foreach ($val as $k=>$field) {
        array_push($form, $k."='".$field."'");
    }
    $sql .= implode(",",$form);
    //echo $sql;
}

Output:

INSERT INTO top SET one='inner one',two='inner two'