---- BEGIN CODE ----
function ifNull($var, $value, $null = NULL) {
    return ($var !== $null)? $var : $value;
}
----- END CODE -----

Or even simpler to use and, unlike previous, working fine with undefined variables:

---- BEGIN CODE ----
function unNull(&$var, $value, $null = NULL) {
    if ($var === $null) {
        $var = $value;
    }
    return $var;
}
----- END CODE -----

If we drop the last argument even multiple operands version can be achieved:
---- BEGIN CODE ----
function nonNull() {
    return array_reduce(func_get_args(), function(&$a, $e) {
        return ($a === NULL && $e !== NULL)? $e : $a;
    });
}
----- END CODE -----
Any reason to not do this in userland?

Do you mean this operator?

$a = $a ?: "default value");

http://www.php.net/manual/en/language.operators.comparison.php#language.operators.
comparison.ternary

Code
----
<?php
// https://bugs.php.net/bug.php?id=62043

$var1 = null;
$var2 = null;
$var3 = $var1 ?: $var2; //$var3==null
$var4 = $var1 ?: 'default1'; //$var4=='default1'
$var5 = $var1 ?: $var2 ?: 'default2'; //$var5=='default2'
$var6 = $var2 ?: $var3 ?: $var4 ?: 'default3'; //$var6=='default1' since the first non-null value is $var4=='default1'

var_dump($var1, $var2, $var3, $var4, $var5, $var6);

----

Already outputs the following on all >= 5.3.0 versions (tested with 5.3.0-5.3.13, 5.4.0-5.4.3) :
----
NULL
NULL
NULL
string(8) "default1"
string(8) "default2"
string(8) "default1"
----
(see http://3v4l.org/rKOqU )

So the ?: operator matches your proposed behaviour.

Thank you for taking the time to report a problem with PHP.
Unfortunately you are not using a current version of PHP -- 
the problem might already be fixed. Please download a new
PHP version from http://www.php.net/downloads.php

If you are able to reproduce the bug with one of the latest
versions of PHP, please change the PHP version on this bug report
to the version you tested and change the status back to "Open".
Again, thank you for your continued support of PHP.

A variation on this did get in: http://wiki.php.net/rfc/isset_ternary