Testing if a variable is empty

bknowles

Hey all, just a quick question on something i've been wondering about for a long while now...

Quite often in my scripts i'll have to check if a variable has a value assigned to it... ie, that it's been set, and has some sort of a value in it. I know of three ways to test this and i usually end up using a mix of them:

if (empty($var)) {
print 'variable is empty';
}

if (!isset($var)) {
print 'variable is empty';
}

if ($var == NULL) {
print 'variable is empty';
}

What are the differences between these three methods? If i just want to check that a variable has a value in it which is the best to use?

usually i use something like this:

if (isset($var) && $var != NULL) {
// do stuff
}

But i've never really been sure if that's the right way to do it, or if there's a better way.

Thanks,

-ben

simulant

NULL and the empty string will convert to FALSE, so you could do this:

if (!$var) {
  // do stuff
}

See the Converting to boolean section in the manual. Interestingly, arrays with no elements and objects without any member variables also evaluate to FALSE.

But if you're referencing an array, you'll get an index error if that index doesn't exist, so do this in that case:

if (!@$arr["index"]) {
  // do stuff
}

Weedpacket

Originally posted by bknowles
Hey all, just a quick question on something i've been wondering about for a long while now...

Try this script; it will show each of five methods in action:

<?php
error_reporting(E_ALL);
$this=0;
$that=42;
$other=null;

echo (empty($this)) ? '$this is empty<br>' : '$this is not empty<br>';
echo (isset($this)) ? '$this is set<br>' : '$this is not set<br>';
echo ($this==null) ? '$this is null<br>' : '$this is not null<br>';
echo ($this===null) ? '$this is really null<br>' : '$this is not really null<br>';
echo (!$this) ? '$this is false<br>' : '$this is not false<br>';
echo '<br>';
echo (empty($that)) ? '$that is empty<br>' : '$that is not empty<br>';
echo (isset($that)) ? '$that is set<br>' : '$that is not set<br>';
echo ($that==null) ? '$that is null<br>' : '$that is not null<br>';
echo ($that===null) ? '$that is really null<br>' : '$that is not really null<br>';
echo (!$that) ? '$that is false<br>' : '$that is not false<br>';
echo '<br>';
echo (empty($other)) ? '$other is empty<br>' : '$other is not empty<br>';
echo (isset($other)) ? '$other is set<br>' : '$other is not set<br>';
echo ($other==null) ? '$other is null<br>' : '$other is not null<br>';
echo ($other===null) ? '$other is really null<br>' : '$other is not really null<br>';
echo (!$other) ? '$other is false<br>' : '$other is not false<br>';
echo '<br>';
echo (empty($foo)) ? '$foo is empty<br>' : '$foo is not empty<br>';
echo (isset($foo)) ? '$foo is set<br>' : '$foo is not set<br>';
echo ($foo==null) ? '$foo is null<br>' : '$foo is not null<br>';
echo ($foo===null) ? '$foo is really null<br>' : '$foo is not really null<br>';
echo (!$foo) ? '$foo is false<br>' : '$foo is not false<br>';

?>

Note that each of the four variables checked behaves differently from the others; the difference between $other and $foo is that the latter triggers PHP usage notices when it's used.