Best way to test for a variable's existence in PHP; isset() is clearly broken

Question 1

From the isset() docs:

isset() will return FALSE if testing a variable that has been set to NULL.

Basically, isset() doesn't check for whether the variable is set at all, but whether it's set to anything but NULL.

Given that, what's the best way to actually check for the existence of a variable? I tried something like:

if(isset($v) || @is_null($v))

(the @ is necessary to avoid the warning when $v is not set) but is_null() has a similar problem to isset(): it returns TRUE on unset variables! It also appears that:

@($v === NULL)

works exactly like @is_null($v), so that's out, too.

How are we supposed to reliably check for the existence of a variable in PHP?

Edit: there is clearly a difference in PHP between variables that are not set, and variables that are set to NULL:

<?php
$a = array('b' => NULL);
var_dump($a);

PHP shows that $a['b'] exists, and has a NULL value. If you add:

var_dump(isset($a['b']));
var_dump(isset($a['c']));

you can see the ambiguity I'm talking about with the isset() function. Here's the output of all three of these var_dump()s:

array(1) {
  ["b"]=>
  NULL
}
bool(false)
bool(false)

Further edit: two things.

One, a use case. An array being turned into the data of an SQL UPDATE statement, where the array's keys are the table's columns, and the array's values are the values to be applied to each column. Any of the table's columns can hold a NULL value, signified by passing a NULL value in the array. You need a way to differentiate between an array key not existing, and an array's value being set to NULL; that's the difference between not updating the column's value and updating the column's value to NULL.

Second, Zoredache's answer, array_key_exists() works correctly, for my above use case and for any global variables:

<?php
$a = NULL;
var_dump(array_key_exists('a', $GLOBALS));
var_dump(array_key_exists('b', $GLOBALS));

outputs:

bool(true)
bool(false)

Since that properly handles just about everywhere I can see there being any ambiguity between variables that don't exist and variables that are set to NULL, I'm calling array_key_exists() the official easiest way in PHP to truly check for the existence of a variable.

(Only other case I can think of is for class properties, for which there's property_exists(), which, according to its docs, works similarly to array_key_exists() in that it properly distinguishes between not being set and being set to NULL.)

Question 2

If the variable you are checking would be in the global scope you could do:

array_key_exists('v', $GLOBALS)

Question 3

You are probably expecting PHP to use the $_POST and $_GET as global variables. PHP used to be setup this way, back in the day, but newer versions require you to explicitly reference these variables.

You could try this:

setdefault($_GET['id'], 0);

function setdefault(&$var, $default="") 
{
   if (!isset($var)) 
   {
      $var = $default;
   }
}

or even more simply (using the ternary operator):

$id = array_key_exists('id', $_GET) ? $_GET['id'] : 0;

Question 4

The hash idea is probably the best. You might have collisions, but they would be exceedingly rare.

Make the hash field a unique key for the table, and catch the duplicate error code. Or use insert ignore or insert replace.

Question 5

You can use get_headers($url)

Example 2 from Manual:

<?php
// By default get_headers uses a GET request to fetch the headers. If you
// want to send a HEAD request instead, you can do so using a stream context:
stream_context_set_default(
    array(
        'http' => array(
            'method' => 'HEAD'
        )
    )
);
print_r(get_headers('http://example.com'));

// gives
Array
(
    [0] => HTTP/1.1 200 OK 
    [Date] => Sat, 29 May 2004 12:28:14 GMT
    [Server] => Apache/1.3.27 (Unix)  (Red-Hat/Linux)
    [Last-Modified] => Wed, 08 Jan 2003 23:11:55 GMT
    [ETag] => "3f80f-1b6-3e1cb03b"
    [Accept-Ranges] => bytes
    [Content-Length] => 438
    [Connection] => close
    [Content-Type] => text/html
)

The first array element will contain the HTTP Response Status code. You have to parse that.

Note that the get_headers function in the example will issue an HTTP HEAD request, which means it will not fetch the body of the URL. This is more efficient than using a GET request which will also return the body.

Also note that by setting a default context, any subsequent calls using an http stream context, will now issue HEAD requests. So make sure to reset the default context to use GET again when done.

PHP also provides the variable $http_response_header

The $http_response_header array is similar to the get_headers() function. When using the HTTP wrapper, $http_response_header will be populated with the HTTP response headers. $http_response_header will be created in the local scope.

If you want to download the content of a remote resource, you don't want to do two requests (one to see if the resource exists and one to fetch it), but just one. In that case, use something like file_get_contents to fetch the content and then inspect the headers from the variable.

Question 6

They should just roll over:

File1.php

<?php
$var1 = "TEST";
?>

File2.php

<?php
include("File1.php");
echo $var1; //Outputs TEST
?>

Best way to test for a variable's existence in PHP; isset() is clearly broken

Answers

php,variables,isset