How to replace "if" statement with a ternary operator ( ? : )?

The

(condition) ? /* value to return if condition is true */ 
            : /* value to return if condition is false */ ;

syntax is not a "shorthand if" operator (the ? is called the conditional operator) because you cannot execute code in the same manner as if you did:

if (condition) {
    /* condition is true, do something like echo */
}
else {
    /* condition is false, do something else */
}

In your example, you are executing the echo statement when the $address is not empty. You can't do this the same way with the conditional operator. What you can do however, is echo the result of the conditional operator:

echo empty($address['street2']) ? "Street2 is empty!" : $address['street2'];

and this will display "Street is empty!" if it is empty, otherwise it will display the street2 address.

PHP 7+

As of PHP 7, this task can be performed simply by using the Null coalescing operator like this :

echo $address['street2'] ?? 'Empty';

Basic True / False Declaration

$is_admin = ($user['permissions'] == 'admin' ? true : false);

Conditional Welcome Message

echo 'Welcome '.($user['is_logged_in'] ? $user['first_name'] : 'Guest').'!';

Conditional Items Message

echo 'Your cart contains '.$num_items.' item'.($num_items != 1 ? 's' : '').'.';

ref: https://davidwalsh.name/php-ternary-examples