Does PHP negation check with `!` coprrespond to `!=` or to `!==`?

问题

In PHP, is

if(!$foo)

equivalent with

if($foo != true)

or with

if($foo !== true)

or is it even something completly different of both?

回答1:

if(!$foo)

is the equivalent to

if($foo != true)

so

$foo = null;
if(!$foo){
 echo "asd";
}

will ouptut "asd"

回答2:

Note that,

== OR != compares the values of variables for equality, type casting as necessary. === OR !== checks if the two variables are of the same type AND have the same value.

This answer will give you better explanation of this concept: https://stackoverflow.com/a/80649/3067928

回答3:

Its not the same

!= is No equal (Returns true if  is not equal)
!== is Not identical  (Returns true if  is not equal , or they are not of the same type)

回答4:

$a != $b

TRUE if $a is not equal to $b after type juggling.

$a !== $b

TRUE if $a is not equal to $b, or they are not of the same type.

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See type juggling in PHP for more info on type juggling.

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Sources : php.net

来源：https://stackoverflow.com/questions/27319742/does-php-negation-check-with-coprrespond-to-or-to