Double not (!!) operator in PHP

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;             FALSEvar $x;                FALSE$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