Remove non-ascii characters from string

A regex replace would be the best option. Using $str as an example string and matching it using :print:, which is a POSIX Character Class:

$str = 'aAÂ';
$str = preg_replace('/[[:^print:]]/', '', $str); // should be aA

What :print: does is look for all printable characters. The reverse, :^print:, looks for all non-printable characters. Any characters that are not part of the current character set will be removed.

Note: Before using this method, you must ensure that your current character set is ASCII. POSIX Character Classes support both ASCII and Unicode and will match only according to the current character set. As of PHP 5.6, the default charset is UTF-8.

You want only ASCII printable characters?

use this:

<?php
header('Content-Type: text/html; charset=UTF-8');
$str = "abqwrešđčžsff";
$res = preg_replace('/[^\x20-\x7E]/','', $str);
echo "($str)($res)";

Or even better, convert your input to utf8 and use phputf8 lib to translate 'not normal' characters into their ascii representation:

require_once('libs/utf8/utf8.php');
require_once('libs/utf8/utils/bad.php');
require_once('libs/utf8/utils/validation.php');
require_once('libs/utf8_to_ascii/utf8_to_ascii.php');

if(!utf8_is_valid($str))
{
  $str=utf8_bad_strip($str);
}

$str = utf8_to_ascii($str, '' );

$clearstring=filter_var($rawstring, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_HIGH);