title: How to flatten array in PHP?
body: <p>I have an array that contains 4 arrays with one value each.</p>
  
  <pre><code>array(4) {
    [0]=&gt;
    array(1) {
      ["email"]=&gt;
      string(19) "test01@testmail.com"
    }
    [1]=&gt;
    array(1) {
      ["email"]=&gt;
      string(19) "test02@testmail.com"
    }
    [2]=&gt;
    array(1) {
      ["email"]=&gt;
      string(19) "test03@testmail.com"
    }
    [3]=&gt;
    array(1) {
      ["email"]=&gt;
      string(19) "test04@testmail.com"
    }
  }
  </code></pre>
  
  <p>What is the best (=shortest, native PHP functions preferred) way to flatten the array so that it just contains the email addresses as values:</p>
  
  <pre><code>array(4) {
    [0]=&gt;
    string(19) "test01@testmail.com"
    [1]=&gt;
    string(19) "test02@testmail.com"
    [2]=&gt;
    string(19) "test03@testmail.com"
    [3]=&gt;
    string(19) "test04@testmail.com"
  }
  </code></pre>