title: PHP merge similar values in an array
body: <p>$arr = (a => array(1,2,3), b => array(1,2,3), c => array(4,5), d=> array(8,9,10), e => array(8,9), f => array(9,10);</p>
  
  <p>I would like to merge similar values, so that I would get:</p>
  
  <ul>
  <li>Problem #1 (similar values, harder):</li>
  </ul>
  
  <p>$new_arr = (a_b => array(1,2,3), c => (4,5), d_e_f => array(8,9,10));</p>
  
  <ul>
  <li>Problem #2 (exactly same values, easier):</li>
  </ul>
  
  <p>$new_arr = (a_b => array(1,2,3), c => (4,5), d => array(8,9,10), e => array(8,9), f => array(9,10));</p>
  
  <p>What's the most effective way to to the above?? May be a hard question to solve :D</p>
  
  <p>Thanks!</p>