Best way to give a variable a default value (simulate Perl ||, ||= )

问题

I love doing this sort of thing in Perl: $foo = $bar || $baz to assign $baz to $foo if $bar is empty or undefined. You also have $foo ||= $bletch which will only assign $bletch to $foo if $foo is not defined or empty.

The ternary operator in this situation is tedious and tiresome. Surely there's a simple, elegant method available in PHP?

Or is the only answer a custom function that uses isset()?

回答1:

PHP 5.3 has a shorthand ?: operator:

$foo = $bar ?: $baz;

Which assigns $bar if it's not an empty value (I don't know how this would be different in PHP from Perl), otherwise $baz, and is the same as this in Perl and older versions of PHP:

$foo = $bar ? $bar : $baz;

But PHP does not have a compound assignment operator for this (that is, no equivalent of Perl's ||=).

Also, PHP will make noise if $bar isn't set unless you turn notices off. There is also a semantic difference between isset() and empty(). The former returns false if the variable doesn't exist, or is set to NULL. The latter returns true if it doesn't exist, or is set to 0, '', false or NULL.

回答2:

In PHP 7 we finally have a way to do this elegantly. It is called the Null coalescing operator. You can use it like this:

$name = $_GET['name'] ?? 'john doe';

This is equivalent to

$name = isset($_GET['name']) ? $_GET['name']:'john doe';

回答3:

Thanks for all the great answers!

For anyone else coming here for a possible alternative, here are some functions that help take the tedium out of this sort of thing.

function set_if_defined(&$var, $test){
    if (isset($test)){
        $var = $test;
        return true;
    } else {
        return false;
    }
}

function set_unless_defined(&$var, $default_var){
    if (! isset($var)){
        $var = $default_var;
        return true;
    } else {
        return false;
    }
}

function select_defined(){
    $l = func_num_args();
    $a = func_get_args();
    for ($i=0; $i<$l; $i++){
        if ($a[$i]) return $a[$i];
    }
}

Examples:

// $foo ||= $bar;
set_unless_defined($foo, $bar);

//$foo = $baz || $bletch
$foo = select_defined($baz, $bletch);

I'm sure these can be improved upon.

回答4:

A common idiom to stay compatible with older PHP versions is:

 $var = $bool   or   $var = "default";
 // If I use it, then only with excessive spaces for clarity.

This works for values that can be evaluated in boolean context. The advantage here is that it also gives you said debug e_notice should the variable be undefined.