PHP 语法问题: 问号和冒号是什么意思？

This is the PHP ternary operator (also known as a conditional operator) - if first operand evaluates true, evaluate as second operand, else evaluate as third operand.

Think of it as an "if" statement you can use in expressions. Can be very useful in making concise assignments that depend on some condition, e.g.

$param = isset($_GET['param']) ? $_GET['param'] : 'default';

There's also a shorthand version of this (in PHP 5.3 onwards). You can leave out the middle operand. The operator will evaluate as the first operand if it true, and the third operand otherwise. For example:

$result = $x ?: 'default';

It is worth mentioning that the above code when using i.e. $_GET or $_POST variable will throw undefined index notice and to prevent that we need to use a longer version, with isset or a null coalescing operator which is introduced in PHP7:

$param = $_GET['param'] ?? 'default';

It's the ternary form of the if-else operator. The above statement basically reads like this:

if ($add_review) then {
return FALSE; //$add_review evaluated as True
} else {
return $arg //$add_review evaluated as False
}

See here for more details on ternary op in PHP: http://www.addedbytes.com/php/ternary-conditionals/