title: PHP merge array on nulls
body: <p>I have 2 * <code>array</code> and I want to <code>merge</code> them, but each of theme have some <code>NULL</code> <code>rows</code>.</p>
  
  <pre><code>$a = array(
      'a' =&gt; NULL,
      'b' =&gt; 1,
      'c' =&gt; 1
  );
  
  $b = array(
      'a' =&gt; 1,
      'b' =&gt; NULL,
      'c' =&gt; 1    
  );
  </code></pre>
  
  <p>So, code:</p>
  
  <pre><code>$c = array_merge($a,$b);
  </code></pre>
  
  <p>Will give $c:</p>
  
  <pre><code>array {
    'a'=&gt; 1
    'b'=&gt; NULL
    'c'=&gt;1
  }
  </code></pre>
  
  <p>Is there build in or simple way to do margin <code>($a,$b)</code> like following, but now <code>$a</code> is overwritten for every same index from <code>$b</code>. I want <code>$b</code> to be overwritten by $a index if <code>$b</code> index is <code>null</code> - in example <code>$b['b']</code> should be overwritten from <code>$a</code></p>
  
  <p>All NULL rows should be filled if possible.</p>