PHP isset() function misfiring

Question

I'm having a problem with PHP's isset function. It's often and mysteriously (to me) misfiring.

For instance, when I have a variable that can be either a string or an error array, I try using isset to see if the variable contains one of the known indexes of the array, like so:

$a = "72";
if(isset($a["ErrorTable"]))
    echo "YES";
else
    echo "NO";

This bad boy prints YES all the way on my server. I tried it on Ideone (online interpreter thingie. It's cool!) here: http://ideone.com/r6QKhK and it prints out NO.

I'm thinking this has something to do with the PHP version we're using. Can someone shed some light into this?

Answer 1

Consider the following piece of code:

$a = "72";
var_dump( isset($a["ErrorTable"]) );

You're checking if $a["ErrorTable"] is set. PHP first typecasts any non-numeric offset to int and this makes ErrorTable equal to 0.

Essentially, you're just doing:

if ( isset($a[0]) ) 

Strings in PHP can be accessed an array, and $a[0] is definitely set and the condition will evaluate to TRUE.

However, this weird behavior was fixed in PHP 5.4.0 and the changelog for isset() says:

5.4.0 -- Checking non-numeric offsets of strings now returns FALSE.

Your server is probably using an older version of PHP and that would explain why it outputs YES.

---

You can use array_key_exists() instead:

$a = "72";
if ( is_array($a) && array_key_exists('ErrorTable', $a) ) {
    echo 'YES';
} else {
    echo 'NO';
}

The output will be NO on all PHP versions.