hexadecimal validation

Part of my project I needed hexadecimal validation so I write these function. You guys may need this too enjoy! Suggestion are welcome.

<?php
// ------------------------------------------------------------------------
/**
 * Hexadecimal validation function
 *
 * Useful for validating hexadecimal value
 *
 * @access    public
 * @param    string     input string.

 * @return    Boolean
 */    
function is_hex($entry)
{
$validChar=array('0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F');// characters allowed in hex
$stringlen=strlen($entry); 
$entry=strtoupper($entry);   // lowercase characters? make uppercase
// Now scan string for illegal characters
for ($i=0;$i<$stringlen;$i++){
    if(!in_array(charAt($entry,$i),$validChar)){        
       return false;}
    } // end scanning loop
return true;
}
/**
 * Javascript simalr charAt function
 *
 * Pick the string from the define position
 * @access    public
 * @param    string     input string.
 * @return    1 Character
 */    
function charAt($str, $pos)
   {
      return (substr($str, $pos, 1));
   }
?&gt;

This helper funcrion can be changed for a simple regexp. So

<?php
// ------------------------------------------------------------------------
/**
 * Hexadecimal validation function
 *
 * Useful for validating hexadecimal value
 *
 * @access    public
 * @param    string     input string.
 * @return    Boolean
 */
function is_hex($entry)
{
  return (bool) preg_match ('/[^0-9a-fA-F]/', $entry);
}
?>