How do the PHP equality (== double equals) and identity (=== triple equals) comparison operators differ?

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What is the difference
between == and
===?

• How
  exactly does the loosely == comparison
  work?


• How exactly does the strict
  === comparison work?
  

What would be some
useful examples?

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class="normal">Answer

The
difference between the loosely == equal operator and the strict
=== identical operator is exactly explained in the href="http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison"
rel="noreferrer">manual:

Comparison
Operators

┌──────────┬───────────┬───────────────────────────────────────────────────────────┐
│
            Example │ Name │ Result
            │
├──────────┼───────────┼───────────────────────────────────────────────────────────┤

│$a
            == $b │ Equal │ TRUE if $a is equal to $b after type juggling. │
│$a === $b │
            Identical │ TRUE if $a is equal to $b, and they are of the same type.
            │
└──────────┴───────────┴───────────────────────────────────────────────────────────┘

/>

If you are using
the == operator, or any other comparison operator which uses
loosely comparison such as !=,
<> or ==, you always have to
look at the context to see what, where and why something
gets converted to understand what is going
on.

Converting
rules

Type comparison
table

As reference and example you can see the
comparison table in the href="http://php.net/manual/en/types.comparisons.php#types.comparisions-loose"
rel="noreferrer">manual:

Loose comparisons with
==

┌─────────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬─────────┬───────┬───────┐
│
            │ TRUE │ FALSE │ 1 │ 0 │ -1 │ "1" │ "0" │ "-1" │ NULL │ array() │ "php" │ ""
            │
├─────────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼─────────┼───────┼───────┤
│
            TRUE │ TRUE │ FALSE │ TRUE │ FALSE │ TRUE │ TRUE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE │
            FALSE │
│ FALSE │ FALSE │ TRUE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE │ FALSE │
            TRUE │ TRUE │ FALSE │ TRUE │
│ 1 │ TRUE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE
            │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ 0 │ FALSE │ TRUE │ FALSE │
            TRUE │ FALSE │ FALSE │ TRUE │ FALSE │ TRUE │ FALSE │ TRUE │ TRUE
            │

│ -1 │ TRUE │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE
            │ FALSE │ FALSE │ FALSE │ FALSE │
│ "1" │ TRUE │ FALSE │ TRUE │ FALSE │ FALSE
            │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ "0" │ FALSE │ TRUE
            │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE
            │
│ "-1" │ TRUE │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE │ FALSE
            │ FALSE │ FALSE │ FALSE │
│ NULL │ FALSE │ TRUE │ FALSE │ TRUE │ FALSE │ FALSE
            │ FALSE │ FALSE │ TRUE │ TRUE │ FALSE │ TRUE │
│ array() │ FALSE │ TRUE │
            FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE │ TRUE │ FALSE │ FALSE
            │
│ "php" │ TRUE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │
            FALSE │ FALSE │ TRUE │ FALSE │
│ "" │ FALSE │ TRUE │ FALSE │ TRUE │ FALSE │
            FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ TRUE
            │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘

If
you are using the === operator, or any other comparison
operator which uses strict comparison such as !== or
===, then you can always be sure that the types won't
magically change, because there will be no converting going on. So
with strict comparison the type and value have to be the same, not only the
value.

Type comparison
table

As reference and example you can see the
comparison table in the href="http://php.net/manual/en/types.comparisons.php#type.comparisons-strict"
rel="noreferrer">manual:

Strict comparisons with
===

┌─────────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬─────────┬───────┬───────┐
│
            │ TRUE │ FALSE │ 1 │ 0 │ -1 │ "1" │ "0" │ "-1" │ NULL │ array() │ "php" │ ""
            │
├─────────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼─────────┼───────┼───────┤
│
            TRUE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
            FALSE │ FALSE │
│ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE
            │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ 1 │ FALSE │ FALSE │ TRUE │ FALSE │
            FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ 0 │ FALSE │
            FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE
            │

│ -1 │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │
            FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ "1" │ FALSE │ FALSE │ FALSE │ FALSE
            │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
│ "0" │ FALSE
            │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE
            │
│ "-1" │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE │
            FALSE │ FALSE │ FALSE │ FALSE │
│ NULL │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE
            │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE │ FALSE │
│ array() │ FALSE │
            FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE │ FALSE │ FALSE
            │
│ "php" │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
            FALSE │ FALSE │ TRUE │ FALSE │
│ "" │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │
            FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE
            │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘