What does the double not operator do in PHP?
For example:
return !! $row;
What would the code above do?
It's not the "double not operator", it's the not operator applied twice. The right ! will result in a boolean, regardless of the operand. Then the left ! will negate that boolean.
This means that for any true value (numbers other than zero, non-empty strings and arrays, etc.) you will get the boolean value TRUE, and for any false value (0, 0.0, NULL, empty strings or empty arrays) you will get the boolean value FALSE.
It is functionally equivalent to a cast to boolean:
return (bool)$row;
It's the same (or almost the same - there might be some corner case) as casting to bool. If $row would cast to true, then !! $row is also true.
But if you want to achieve (bool) $row, you should probably use just that - and not some "interesting" expressions ;)
It means if $row has a truthy value, it will return true, otherwise false, converting to a boolean value.
Here is example expressions to boolean conversion from php docs.
Expression Boolean
$x = ""; FALSE
$x = null; FALSE
var $x; FALSE
$x is undefined FALSE
$x = array(); FALSE
$x = array('a', 'b'); TRUE
$x = false; FALSE
$x = true; TRUE
$x = 1; TRUE
$x = 42; TRUE
$x = 0; FALSE
$x = -1; TRUE
$x = "1"; TRUE
$x = "0"; FALSE
$x = "-1"; TRUE
$x = "php"; TRUE
$x = "true"; TRUE
$x = "false"; TRUE
"not not" is a convenient way in many languages for understanding what truth value the language assigns to the result of any expression. For example, in Python:
>>> not not []
False
>>> not not [False]
True
It can be convenient in places where you want to reduce a complex value down to something like "is there a value at all?".
Another more human, maybe simpler, way to 'read' the not not:
The first '!' does 2 things: 'convert' the value to boolean, then output its opposite. So it will give true if the value is a 'falsy' one.
The second '!' is just to output the opposite of the first.
So, basically, the input value can be anything, maybe a string, but you want a boolean output, so use the first '!'. At this point, if you want TRUE when the input value is 'falsy', then stop here and just use a single '!'; otherwise if you want TRUE when the input value is 'truthy', then add another '!'.