title: preg_replace - only letters and replace spaces for plus character
body: <p>I would like to accept in string <strong>PHP</strong> only letters and change all spaces for plus (+) character. Further I would like to accept only one plus next to each other. So this string:
  <code>$string = 'ex,a-mpl*e   s++tri  ng';</code>
  sholud replace for:
  <code>$string = 'example+s+tri+ng';</code></p>
  
  <p>I tried like that:
  <code>#[^a-zA-Z0-9\+\]#</code>
  but that isn't helpful for me.</p>
  
  <p>Can you help me? :)</p>
  
  <p>EDIT:</p>
  
  <p>aahh, and I would like to also accept only strings where string's lenth is 3 or more.</p>