[php] php is null or empty?

I have a question regarding NULL in PHP:

  $a = '';
  if($a == NULL) {
      echo 'is null';
  }

Why do I see is null when $a is an empty string? Is that a bug?

Just to addon if someone is dealing with  , this would work if dealing with  .

Replace it with str_replace() first and check it with empty()

empty(str_replace(" " ,"" , $YOUR_DATA)) ? $YOUR_DATA = '--' : $YOUR_DATA;

If you use ==, php treats an empty string or array as null. To make the distinction between null and empty, either use === or is_null. So:

if($a === NULL) or if(is_null($a))

This is not a bug but PHP normal behavior. It happens because the == operator in PHP doesn't check for type.

'' == null == 0 == false

If you want also to check if the values have the same type, use === instead. To study in deep this difference, please read the official documentation.

As is shown in the following table, empty($foo) is equivalent to $foo==null and is_null($foo) has the same function of $foo===null. The table also shows some tricky values regarding the null comparison. (? denotes an uninitialized variables. )

         empty    is_null 
         ==null  ===null  isset   array_key_exists
      ? |   T   |   T   |   F   |   F   
   null |   T   |   T   |   F   |   T   
     "" |   T   |   F   |   T   |   T   
     [] |   T   |   F   |   T   |   T
      0 |   T   |   F   |   T   |   T      
  false |   T   |   F   |   T   |   T   
   true |   F   |   F   |   T   |   T   
      1 |   F   |   F   |   T   |   T   
     \0 |   F   |   F   |   T   |   T   

NULL stands for a variable without value. To check if a variable is NULL you can either use is_null($var) or the comparison (===) with NULL. Both ways, however, generate a warning if the variable is not defined. Similar to isset($var) and empty($var), which can be used as functions.

var_dump(is_null($var)); // true
var_dump($var === null); // true
var_dump(empty($var)); // true

Read more in How to check if a variable is NULL in PHP?

No it's not a bug. Have a look at the Loose comparisons with == table (second table), which shows the result of comparing each value in the first column with the values in the other columns:

    TRUE    FALSE   1       0       -1      "1"     "0"     "-1"    NULL    array() "php"   ""

    [...]    

""  FALSE   TRUE    FALSE   TRUE    FALSE   FALSE   FALSE   FALSE   TRUE    FALSE   FALSE   TRUE

There you can see that an empty string "" compared with false, 0, NULL or "" will yield true.

You might want to use is_null ^[docs] instead, or strict comparison (third table).

check == vs ===

'' == NULL would return true
0 == NULL would return true
false == null would return true

where as

'' === NULL would return false
0 === NULL would return false
false === NULL would return false

PHP 7 isset() vs empty() vs is_null()

Use empty - http://php.net/manual/en/function.empty.php.

Example:

$a = '';
if(empty($a)) {
    echo 'is empty';
}