How to use HTMLSpecialChars function to check ALL data input fields in PHP?

I have set up the following server side code to validate new user form input. I'm not done with it yet (still need to validate email etc - BTW it is a low traffic site so using server side scripting is not an issue). Before I go further with it I would like toknow how to  have ALL data fields checked for "malicious scripters" by using the HTMLSpecialChars() function. How can I do that with out adding the test individually to all of the fields below - ie how can I have it automatically check any data entry field? Thanks in advance for any help (yes, I'm a struglling beginner :-)

//create short variables
$username = strtoupper(ltrim($_POST['username']));
$firstname = ucfirst (ltrim($_POST['firstname']));
$lastname = ucfirst (ltrim($_POST['lastname']));
$address = ltrim($_POST['address']);
$address2 = ltrim($_POST['address2']);
$city = ltrim($_POST['city']);
$zip = ltrim($_POST['zip']);
$email = strtolower(ltrim($_POST['email']));
$homephone = ltrim($_POST['homephone']);
$workphone = ltrim($_POST['workphone']);
$cellphone = ltrim($_POST['cellphone']);
$password = ltrim($_POST['password']);
$password_ck = ltrim($_POST['password_ck']);

// Test whether the form has been submitted in the first place
if (isset($_POST['Submit']))
  {
   // Test whether your required field is blank
   if (empty($username))
              {
              $ERRusername = TRUE;
              $ERRusernameMsg = "You must enter member name";
              $success = FALSE;
              }              
   if (empty($firstname))
          {
           $ERRfirstname = TRUE;
           $ERRfirstnameMsg = "You must enter first name";
           $success = FALSE;            
          }
    //first name validation              
      if (!eregi("^[a-z]*$",$firstname))      
            {
           $ERRfirstname_alpha = TRUE;
           $ERRfirstname_alphaMsg = "Use only alpha characters.";
           $success = FALSE;            
          }         
   if (empty($lastname))
          {
           $ERRlastname = TRUE;
           $ERRlastnameMsg = "You must enter last name";
           $success = FALSE;            
          }      
      //last name validation              
      if (!eregi("^[a-z]*$",$lastname))      
            {
           $ERRlastname_alpha = TRUE;
           $ERRlastname_alphaMsg = "Use only alpha characters.";
           $success = FALSE;            
          }       
   if (empty($address))
          {
           $ERRaddress = TRUE;
           $ERRaddressMsg = "You must enter an address";
           $success = FALSE;            
          }                  
      if (empty($city))
          {
           $ERRcity = TRUE;
           $ERRcityMsg = "You must enter a city";
           $success = FALSE;            
          }  
      if (empty($zip))
          {
           $ERRzip = TRUE;
           $ERRzipMsg = "You must enter zipcode";
           $success = FALSE;            
          }
//zip code validation                
      if (!ereg("^([0-9]{5})$",$zip))      
               {
           $ERRzip_int = TRUE;
           $ERRzip_intMsg = "Zipcode must contain 5 digits.";
           $success = FALSE;            
          }    
      if (empty($email))
          {
           $ERRemail = TRUE;
           $ERRemailMsg = "You must enter email address";
           $success = FALSE;            
          }
       if (empty($homephone))
          {
           $ERRhomephone = TRUE;
           $ERRhomephoneMsg = "You must enter your home phone";
           $success = FALSE;            
          }           
       if (empty($password))
          {
           $ERRpassword = TRUE;
           $ERRpasswordMsg = "You must enter password";
           $success = FALSE;            
          }  
        if (strlen($password)>12 OR strlen($password)<5)  
              {
           $ERRpassword_len = TRUE;
           $ERRpassword_lenMsg = "Password must be between 5 and 12 characters.  Try again";
           $success = FALSE;            
          }           
       if (empty($password_ck))
          {
           $ERRpassword_ck = TRUE;
           $ERRpassword_ckMsg = "You must re-enter password";
           $success = FALSE;            
          }             
        if (($password)!=($password_ck))
              {
           $ERRpasswords_not_eq = TRUE;
           $ERRpasswords_not_eqMsg = "Passwords entered were not the same.";
           $success = FALSE;            
          }              
             }// End of POST test  
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