Tests does search for an ascii char with strpos works in utf-8 strings.

create_all_utf.php

<?php

/** @link http://stackoverflow.com/questions/2748956/how-would-you-create-a-string-of-all-utf-8-characters */
function unichr($i)
{
    return iconv('UCS-4LE', 'UTF-8', pack('V', $i));
}

$codeunits = array();
for ($i = 0; $i<0xD800; $i++) {
    $codeunits[] = unichr($i);
}
for ($i = 0xE000; $i<0xFFFF; $i++) {
    $codeunits[] = unichr($i);
}
$all = implode($codeunits);
file_put_contents('all_utf8.txt', $all);

test_strpos.php

<?php

function test($data, $char) {
    $pos = 0;
    $mbpos = 0;
    $count = 0;
    $mbcount = 0;
    while (($mbpos = mb_strpos($data, $char, $mbpos + 1, 'UTF-8')) !== false) {
        $mbcount++;
    }
    while (($pos = strpos($data, $char, $pos + 1)) !== false) {
        $count++;
    }
    assert($mbcount === $count);
    echo 'Tested '.mb_strlen($data, 'UTF-8').' chars and '.strlen($data).' bytes of data! Found: '.$mbcount.' occurences of '.$char.'.'."\n";
}

$files = [];
if (is_dir($argv[1])) {
    $dh = opendir($argv[1]);
    while (($file = readdir($dh)) !== false) {
        if (strpos($file, '.') !== 0) {
            $files[] = $argv[1] . '/' . $file;
        }
    }
    closedir($dh);
} else {
    $files = [$argv[1]];
}

foreach ($files as $file) {
    test(file_get_contents($file), $argv[2]);
}