What is the meaning of the '?', '()', and ':' symbols in PHP?

问题

I've finally remembered what to ask. I never really got what : and ? do when a variable is being defined like this:

$ip = ($_SERVER['HTTP_X_FORWARDED_FOR']) ? $_SERVER['HTTP_X_FORWARDED_FOR'] : $_SERVER['REMOTE_ADDR'];

As you can see there is ? and : and ( )

Could anyone give me a brief detail about why and how they are used for?

回答1:

The expression looks like this:

$var = (condition) ? if_true : if_false

?: is the ternary operator. If condition is true, $var will be assigned the value if_true; otherwise it will be assigned the value if_false.

In your particular case:

This assigns the value of the X-Forwarded-For HTTP header to $ip if it exists; otherwise it uses the remote address itself.
This is usually used as a way to get a client's IP address. However, note that in general this is a terrible way to check for client identity. See this StackOverflow question. (Use session cookies or some sort of authentication if you need to make sure users don't clobber each other.)
Also, it's HTTP_X_FORWARDED_FOR, not HTTP_X_FORWARD_FOR.
Finally, HTTP_X_FORWARDED_FOR can be a comma-delimited list of IP addresses, not just a single one, so this has the potential to be a bug.

回答2:

It's known as a ternary operator and is shorthand for (in your case):

if($_SERVER['HTTP_X_FORWARD_FOR'])
{
    $ip = $_SERVER['HTTP_X_FORWARD_FOR'];
}
else
{
    $ip = $_SERVER['REMOTE_ADDR'];
}

回答3:

"?:" (or ternary) operator

The expression (expr1) ? (expr2) : (expr3) evaluates to expr2 if expr1 evaluates to TRUE, and expr3 if expr1 evaluates to FALSE

See this example:

<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];

// The above is identical to this if/else statement
if (empty($_POST['action'])) {
    $action = 'default';
} else {
    $action = $_POST['action'];
}

?>

回答4:

The ternary form is basically a shortcut for if->then->else

I generally avoid it because it's not all that readable.

$ip = ($_SERVER['HTTP_X_FORWARD_FOR']) ? $_SERVER['HTTP_X_FORWARD_FOR'] : $_SERVER['REMOTE_ADDR'];

is logically equivalent to:

if($_SERVER['HTTP_X_FORWARD_FOR']){
   $ip = $_SERVER['HTTP_X_FORWARD_FOR'];
}else{
   $ip = $_SERVER['REMOTE_ADDR'];
}

It should be said that this is EXACTLY what this is most commonly used for: variable initialization. Very common with form data.

来源：https://stackoverflow.com/questions/3101936/what-is-the-meaning-of-the-and-symbols-in-php

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variables

definition