The following code will produce the correct results (with a probable speed increase).

/*
*
* Decoding  Quoted-printable string.
*
*** modified 2000.06.30 by Mark Peters
*** fixed code to recognize '= cr lf' sequence as soft newline
*** and ignore (hide) mis-formed or bogus escape sequences
*/

/* {{{ proto string quoted_printable_decode(string str)
   Convert a quoted-printable string to an 8 bit string */

PHP_FUNCTION(quoted_printable_decode)
{
	pval **arg1;
	char *str;
	int i = 0, j = 0;

	
    if (ZEND_NUM_ARGS() != 1 || zend_get_parameters_ex(1,&arg1)==FAILURE) 
    {
    	WRONG_PARAM_COUNT;
    }
    convert_to_string_ex(arg1);
    
    str = (*arg1)->value.str.val;
    while ( str[i] )
    {
      if (str[i] == '=')   // start of escape sequence
      {
         // look for soft line break sequence ( = cr lf )
         if (str[i+1] == 13 && str[i+2] == 10)
         {
             i += 3;	// skip over crlf sequence
             continue;
         }

         // first digit not hex?
         if (!isxdigit((int)str[i+1]))
         {
             ++i;
             continue;
         }
         // second digit not hex?
         if (!isxdigit((int)str[i+2]))
         {
             i += 2;
             continue;
         }

         // good escape code
         str[j++] = (php_hex2int((int)str[i+1]) << 4 ) + php_hex2int((int)str[i+2]);
         i += 3;
         continue;
      } 

      str[j++] = str[i++];

    }

    str[j] = '\0';
    
    RETVAL_STRINGL(str, j, 1)
}