$foobar   =   Array('firstname'=>'peter','age'=>'32','nation'=>'');
$feebar   =   Array('firstname' => '','lastname' => 'griffin', age =>'33','nation'=>'usa');

print_r(array_merge($foobar,$feebar));
/*
Array
(
    [firstname] =>           // <-- feebar set this to blank, NOT COOL!
    [age] => 33              // <-- feebar set this to 33, thats cool
    [lastname] => griffin    // <-- feebar added this key-value pair, thats cool
    [nation] => usa          // <-- feebar filled in a blank, thats cool.
)
*/

print_r(array_coolmerge($foobar,$feebar));
/*
Array
(
    [firstname] => peter  // <-- don't blank out a value if one already exists!
    [age] => 33
    [lastname] => griffin
    [nation] => usa

)
*/

$merged = array_merge(array_filter($foo, 'strval'), array_filter($bar, 'strval'));

$merged = array_map(
    create_function('$foo,$bar','return ($bar?$bar:$foo);'),
    $foobar,$feebar
);

# Replace keys in $foo
foreach ($foo as $key => $value) {
    if ($value != '' || !isset($bar[$key])) continue;
    $foo[$key] = $bar[$key];
}

# Add other keys in $bar
# Will not overwrite existing keys in $foo
$foo += $bar;

<?php
  $foobar =   Array('firstname' => 'peter','age' => '33',);
  $feebar =   Array('firstname' => '','lastname' => 'griffin',);
  $merged=$foobar;
  foreach($feebar as $k=>$v){
    if(isset($foobar[$k]))$merged[$k]=array($v,$foobar[$k]);
    else $merged[$k]=$v;
  }
  print_r($merged);
?>

<?php
  $foobar =   Array('firstname' => 'peter','age' => '33',);
  $feebar =   Array('firstname' => '','lastname' => 'griffin',);
  $merged=$foobar;
  foreach($feebar as $k=>$v) if($v)$merged[$k]=$v;
  print_r($merged);
?>

<?
  function cool_merge($array1,$array2){
    $result=$array1;
    foreach($array2 as $k=>$v) if($v)$result[$k]=$v;
    return $result;
  }

  $foobar =   Array('firstname' => 'peter','age' => '33',);
  $feebar =   Array('firstname' => '','lastname' => 'griffin',);
  print_r(cool_merge($foobar,$feebar));
?>

array_filter($foo) + array_filter($bar) + $foo + $bar

array_merge($feebar,$foobar)