Does variables' order matter when doing == and === comparisons? [duplicate]

Answer 1

I think that the following code will explain it:

echo (( 0 == NULL ) ? "0 is null" : "0 is not null")  ."\n";
echo  (0 === NULL) ? "0 is null" : "0 is not null"  ;

it will output:

0 is null
0 is not null

The === operator checks for both value and type, but 0 is a number while NULL is of type NULL

The == operator checks only for value and zero can be casted to "false" or "null" hence the "truthy" that we get for the first line

Answer 2

Compare results from this to ifs.

 echo "Test '1abc' == 1 - ";
 if ('1abc' == 1) {
    echo 'ok';
 } else {
    echo 'fail';
 }

 echo "\nTest '1abc' === 1 - ";
 if ('1abc' === 1) {
    echo 'ok';
 } else {
    echo 'fail';
 }

Then read this page http://php.net/manual/en/language.types.type-juggling.php it's very interesting lecture ;)

Answer 3

It prevents typos which cause very hard to debug bugs. That's in case you accidentally write = instead of ==:

if ($var == null)  # This is true in case $var actually is null
if ($var =  null)  # This will always be true, it will assign $var the value null

Instead, switching them is safer:

if (null == $var)   # True if $var is null
if (null =  $var)   # This will raise compiler (or parser) error, and will stop execution.

Stopping execution on a specific line with this problem will make it very easy to debug. The other way around is quite harder, since you may find yourself entering if conditions, and losing variable's values, and you won't find it easily.