Replace invalid characters with a symbol instead of returning empty string for the

entire to be encoded string.

Currently, if an email contains such an invalid character, the whole line will be
left out, which is rather confusing. Even more when it's the email subject. This
constant is only available in PHP 5.4 and up.

functions/strings.php

@@ -1685,7 +1685,7 @@ function sm_validate_security_token($token, $validity_period=0, $show_error=FALS
   * @param string $string The string to be converted
   * @param int $flags A bitmask that controls the behavior of htmlspecialchars()
   *                   (See http://php.net/manual/function.htmlspecialchars.php )
-  *                   (OPTIONAL; default ENT_COMPAT)
+  *                   (OPTIONAL; default ENT_COMPAT, ENT_COMPAT | ENT_SUBSTITUTE for PHP >=5.4)
   * @param string $encoding The character encoding to use in the conversion
   *                         (OPTIONAL; default automatic detection)
   * @param boolean $double_encode Whether or not to convert entities that are
@@ -1706,9 +1706,14 @@ function sm_encode_html_special_chars($string, $flags=ENT_COMPAT,
       $encoding = $default_charset;
    }
 
-// TODO: Is adding this check an unnecessary performance hit?
-   if (check_php_version(5, 2, 3))
+   if (check_php_version(5, 2, 3)) {
+      // Replace invalid characters with a symbol instead of returning
+      // empty string for the entire to be encoded string.
+      if (check_php_version(5, 4, 0) && $flags == ENT_COMPAT) {
+         $flags = $flags | ENT_SUBSTITUTE;
+      }
       return htmlspecialchars($string, $flags, $encoding, $double_encode);
+   }
 
    return htmlspecialchars($string, $flags, $encoding);
 }