difference between is_null “== NULL” and “=== NULL” in PHP [duplicate]

问题

Possible Duplicate:
php == vs === operator

i have the following code fragment and it doesn't make sense to me why would NULL be evaluated in 3 different ways. Consider the variable $uploaded_filenames_array as UNKNOWN - we don't know whether it's still an array or a NULL. That's what we are trying to check.

//-----------------------------------------------
if (is_null($uploaded_filenames_array)){
    echo "is_null";
}
else{
    echo "is_NOT_null";
}
//-----------------------------------------------
if ($uploaded_filenames_array == NULL){
    echo "NULL stuff";
}
else{
    echo "not NULL stuff";
}
//-----------------------------------------------
if ($uploaded_filenames_array === NULL){
    echo "NULL identity";
}
else{
    echo "not NULL identity";
}
//-----------------------------------------------

i am getting the following response:

is_NOT_null 
NULL stuff 
not NULL identity

can somebody help to understand what is the programmatic difference between these 3 ways of checking NULL?

回答1:

is_null($a) is same as $a === null.

($a === null is bit faster than is_null($a) for saving one function call, but it doesn't matter, just choose the style you like.)

For the difference of === and ==, read PHP type comparison tables

$a === null be true only if $a is null.

But for ==, the below also returns true.

null == false
null == 0
null == array()
null == ""

回答2:

You should read this http://php.net/manual/en/language.operators.comparison.php. Also no need to use is_null function to check only on NULL. === is faster...

回答3:

The === operator tests for the same value and the same TYPE. An empty string might evaluate to null, but it is not of the null type - hence this fails.

The == operator basically checks to see if they are pretty much the same - by that, do the evaluate to the same value. Being empty, this will evaluate to null, hence this fails.

The is_null function does a fairly thorough check - much more like the === operator.