PHP: Check if variable exist but also if has a value equal to something

Question

I have (or not) a variable $_GET['myvar'] coming from my query string and I want to check if this variable exists and also if the value corresponds to something inside my if statement:

What I'm doing and think is not the best way to do:

if(isset($_GET['myvar']) && $_GET['myvar'] == 'something'): do something

My question is, exist any way to do this without declare the variable twice?

That is a simple case but imagine have to compare many of this $myvar variables.

Solution 1

Sadly that's the only way to do it. But there are approaches for dealing with larger arrays. For instance something like this:

$required = array('myvar', 'foo', 'bar', 'baz');
$missing = array_diff($required, array_keys($_GET));

The variable $missing now contains a list of values that are required, but missing from the $_GET array. You can use the $missing array to display a message to the visitor.

Or you can use something like that:

$required = array('myvar', 'foo', 'bar', 'baz');
$missing = array_diff($required, array_keys($_GET));
foreach($missing as $m ) {
    $_GET[$m] = null;
}

Now each required element at least has a default value. You can now use if($_GET['myvar'] == 'something') without worrying that the key isn't set.

Update

One other way to clean up the code would be using a function that checks if the value is set.

function getValue($key) {
    if (!isset($_GET[$key])) {
        return false;
    }
    return $_GET[$key];
}

if (getValue('myvar') == 'something') {
    // Do something
}

Solution 2

As of PHP7 you can use the Null Coalescing Operator ?? to avoid the double reference:

// $_GET['myvar'] isn't set...
echo ($_GET['myvar'] ?? '') == 'hello' ? "hello!\n" : "goodbye!\n";

// $_GET['myvar'] is set but != 'hello'
$_GET['myvar'] = 'farewell';
echo ($_GET['myvar'] ?? '') == 'hello' ? "hello!\n" : "goodbye!\n";

// $_GET['myvar'] is set and == 'hello'
$_GET['myvar'] = 'hello';
echo ($_GET['myvar'] ?? '') == 'hello' ? "hello!\n" : "goodbye!\n";

Output:

goodbye!
goodbye!
hello!

Code demo on 3v4l.org

In general, the expression

$a ?? $b

is equivalent to

isset($a) ? $a : $b

Note that in the code example it is necessary to place parentheses around $_GET['myvar'] ?? '' as == has higher precedence than ?? and thus

$_GET['myvar'] ?? '' == 'hello'

would evaluate to:

$_GET['myvar'] ?? ('' == 'hello')

which would be true as long as $_GET['myvar'] was set and "truthy" (see the manual) and false otherwise (since '' == 'hello' is false).

Precedence code demo on 3v4l.org

Solution 3

If you're looking for a one-liner to check the value of a variable you're not sure is set yet, this works:

if ((isset($variable) ? $variable : null) == $value) { }

The only possible downside is that if you're testing for true/false - null will be interpreted as equal to false.

Solution 4

As mellowsoon suggest, you might consider this approach:

required = array('myvar' => "defaultValue1", 'foo' => "value2", 'bar' => "value3", 'baz' => "value4");
$missing = array_diff($required, array_keys($_GET));
foreach($missing as $key => $default  ) {
    $_GET[$key] = $default  ;
}

You put the default values and set the not recieved parameters to a default value :)

View more solutions

Comments

[Your Common Sense]

PHP doesn't have a solution for this, but it's a programming language. You can (and ought to) always write a subprogram to shorten a repetitive code. Not to mention that in a good program every variable should be defined before use...

[Your Common Sense]

Just define all your variables. That's the point of all that mess.

[Mariz Melo]

Well I saw this around there, just hoping be possible without using arrays, thanks.

[Mariz Melo]

Thank you Pekka, is really very boring do that.

[Mariz Melo]

Sometimes in a big system is difficult predict when the variable will appear, that why declare the variable may not help. But you're right in most of the cases.

[Pekka]

@Mariz I disagree: It should never be difficult to predict when the variable will appear: If that is the case, you have bad code.

[mellowsoon]

Updated my answer to show a possible way to do this without using another array.

[Mariz Melo]

Pekka this is the "ideal", but that is difficult in a rush world with people from different background working in the same project. Anyway thanks people.

[Keegan Crain]

And it fires a notice, if the myvar index doesn't exist.

[DanMan]

True, but you're just testing for that index. IMHO it would only be really bad, if that if-clause didn't exist at all. A notice i can live with. After all, it's just a notice, which by definition are harmless (usually).

[Ross Snyder]

Oof, I couldn't disagree more. If your goal is to create a sloppy PHP codebase full of sneaky/silent bugs, ignoring notices would be a great way to start. You should always develop with notices turned on, and you should always treat them as bugs to be fixed.

[Matt Browne]

Unfortunately this triggers a PHP notice if the array key doesn't exist, does it not?

[Dan H]

Nope. This is comparing TRUE/FALSE against 'something'.

[trincot]

It is better to suggest === than ==, as it will not have the downside you mention.

[kmoney12]

this will still fire an error notice when you call _FX('param')

[Taku Yoshi]

If you use PHP7 it can be simpler if (($variable ?? null) === $value).

[RedDragonWebDesign]

Upvote the below answer that mentions ($var ?? 'value'). That's the most concise way to do it now thanks to PHP 7.

[HaLeiVi]

What would happen if I omit the parenthesis?

[Nick]

@HaLeiVi because of operator precedence (== is higher than ??), $_GET['myvar'] ?? '' == 'hello' would evaluate as $_GET['myvar'] ?? ('' == 'hello') so it would evaluate to true if either $_GET['myvar'] is unset (since 'hello' is "truthy") or if $_GET['myvar'] is a "truthy" value (see php.net/manual/en/language.types.boolean.php)

[iWasCloud]

this will always be false, code inside if-block will never run.