title: Phone number validating in php
body: <p>I would like to "validate" my posted phone number. I don't really care about the format, i just want to use only numbers and some chars.
  I tried this code, but if i type at least one number to my string then string will be valid. (for ex.: asdafadas-1asd will be valid)
   How to fix this?</p>
  
  <pre><code>$phonebool=true;
         if (!(strcspn($_POST['phone'], '0123456789-/ ') != strlen($_POST['phone']) )){
             $_SESSION['phone_err']='Only numbers and -/';
             $phonebool=false;
         }
  </code></pre>
  
  <p>thank you.</p>