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

Question

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

What would be some useful examples?

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Additionally, how are these operators used in JavaScript? Are they related to PHP?

Answer 1

Difference between == and ===

The difference between the loosely == equal operator and the strict === identical operator is exactly explained in the 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. │
└──────────┴───────────┴───────────────────────────────────────────────────────────┘

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Loosely == equal comparison

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

• Converting to boolean
• Converting to integer
• Converting to float
• Converting to string
• Converting to array
• Converting to object
• Converting to resource
• Converting to NULL

Type comparison table

As reference and example you can see the comparison table in the 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  │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘

Strict === identical comparison

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 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  │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘