firstset, firstfull, setor, fullor userland implementation.

gistfile1.php

<?php
// Errors on.
error_reporting(E_ALL);
ini_set('display_errors', 1);

/**
 * Returns the first passed argument that is set.
 *
 * @return mixed
 */
function firstset(&$v0=null, &$v1=null, &$v2=null, &$v3=null, &$v4=null, 
&$v5=null, &$v6=null, &$v7=null) {
    foreach (func_get_args() as $arg) {
        if(isset($arg)) {
            return $arg;
        }
    }
    return null;
}

/**
 * Returns the first passed argument that is not empty.
 *
 * @return mixed
 */
function firstfull(&$v0=null, &$v1=null, &$v2=null, &$v3=null, &$v4=null, 
&$v5=null, &$v6=null, &$v7=null) {
    foreach (func_get_args() as $arg) {
        if(!empty($arg)) {
            return $arg;
        }
    }
    return none;
}

/**
 * Sets the passed value by reference to itself if it is set, otherwise to the
 * default value.
 *
 * @param mixed $value
 * @param mixed $default
 */
function setor(&$value, $default) {
    $value = isset($value) ? $value : $default;
    return $value;
}

/**
 * Sets the passed value by reference to itself if it is not empty, otherwise
 * to the default value.
 *
 * @param mixed $value
 * @param mixed $default
 */
function fullor(&$value, $default) {
    $value = !empty($value) ? $value : $default;
    return $value;
}

/**
 *  Short for "as reference". Makes passed argument referenceable.
 * 
 * @param mixed $v
 * @return mixed
 */
function &ar($v) {
    return $v;
}


/**
 * Example code below, solves the goal from https://gist.github.com/909711.
 */

// Dummy function and class.
function foo() { return null; }

class Bar {
    function get() { return null; }
    static function get2() { return null; }
}
$bar = new Bar();

// Set one variable from another or a default value.
// 1a.
$a = firstset($o, ar('Default value 1'));
// 1b.
$b = firstfull($p, ar('Default value 2'));

// Set one variable from itself or a default value.
// 2a.
setor($c, 'Default value 3');
// 2b.
fullor($d, firstfull($z, ar('Default value 4')));

// Find the first set value from a range of variables and statements.
// 3a.
$e = firstset($s, ar(null), ar(foo()), ar($bar->get()), ar(Bar::get2()),
        $t['key']['key2']['key3'], $u->key1->key2->key3, ar('Default value 5'));
// 3b.
$f = firstfull($v, ar(0), ar(foo()), ar($bar->get()), ar(Bar::get2()),
        $x['key']['key2']['key3'], $y->key1->key2->key3, ar('Default value 6'));

var_dump(compact(range('a', 'f')));

/*
Outputs:
array(6) {
  ["a"]=>
  string(15) "Default value 1"
  ["b"]=>
  string(15) "Default value 2"
  ["c"]=>
  string(15) "Default value 3"
  ["d"]=>
  string(15) "Default value 4"
  ["e"]=>
  string(15) "Default value 5"
  ["f"]=>
  string(15) "Default value 6"
}
*/