Question

I am making an ajax call in jquery

$.get("validate_isbn.php", {isbn: obj[16]},
                                        function(answer) 
                                        {                   
                                            console.log(answer);
                                            if (answer == "valid")
                                            {
                                                var checked1 = $(elements[index]).val();
                                                //$(elements[index]).val().append("<img src = 'pics/green_checkmark.png'>");    //doesn't work
                                                //elements.after("<img src = 'pics/green_checkmark.png'>");     //sets all the elements with this pic
                                                elements.eq(index).after("<img src='pics/green_checkmark.png' id='checkmark'>");
                                                var checked = $(elements[index]).val();
                                            }                                                                               
                                        });

which worked fine. I saw in the debugger that it was properly sending over the variable isbn with a isbn number from the obj array. My problem is on the php side. When I was testing, I simply had the code echo “valid” and everything worked out fine. But now when I put the real code in it stopped working:

<?php
//This algorithm is for ISBN 10
function is_isbn_10_valid($n){
$check = 0;
for ($i = 0; $i < 9; $i++) 
{
    $check += (10 - $i) * substr($n, $i, 1);    //starting at the leftmost digit, multiple each digit by a constant, starting at 10, add the total
}   
$t = substr($n, 9, 1); // tenth digit (aka checksum or check digit)
$check += ($t == 'x' || $t == 'X') ? 10 : $t;       //now add the tenth digit
return $check % 11 == 0;
}

//The algorithm for ISBN 13 validation is as follows:
//Multiply each digit of teh isbn, starting at the left, with 1,3,3... etc for the entire isbn (including the check digit becuase its 
//just going to be multiplied by 1 anyways.
//Add them all together, do mod 10 and voila!
function is_isbn_13_valid($n){
$check = 0;
for ($i = 0; $i < 13; $i+=2) //this does digits 1,3,5,7,9,10,11,13
{
    $check += substr($n, $i, 1);
}   
for ($i = 1; $i < 12; $i+=2) //this does digits 2,4,6,8,10,12
{
    $check += 3 * substr($n, $i, 1);
}   
return $check % 10 == 0;
}
$isbn = $_GET["isbn"];
if (strlen($isbn) = 10)
{
$result = is_isbn_10_valid($isbn);
}
else if (strlen($isbn) = 13) 
{
$result = is_isbn_13_valid($isbn);
}
else
{
$result false;
}
if ($result === true)
{echo "valid";}
else if ($result === false)
{echo "not valid";}
?>

(Note: I’m sure that I can be more efficient and just return the boolean, but I refrained from that at the moment since I wasn’t sure how the jquery .get would take it, as a boolean or text…)
Anyways, it doesn’t work. The error on the console.log gives me:

Fatal error: Can’t use function return value in write context in …pathname here…\validate_isbn.php on line 31

Answer 1

On line 31, you have this:

if ($strlen($isbn) = 10)

Remove the $ on the strlen function, and change the = (assignment operator) to a == (equivalence operator). It should now look like this:

if (strlen($isbn) == 10)

You’ll need to do the same a few lines after that, too.

Edit: One more thing. About five lines from the bottom, you’re missing an equal sign.

$result false;

Should be:

$result = false;