title: In PHP, make every letter left of colon lowercase w/o use of explode,implode
body: <p>Can this be done with regular expressions?</p>
  
  <p>Examples</p>
  
  <p>x-example-HEADER:teSt
  becomes
  x-example-header:teSt</p>
  
  <p>y-exaMPLE:testoneTWOthree
  becomes
  y-example:testoneTWOthree</p>