<?php
 
validate_string_chars("aaαα");
 
function validate_string_chars($string) {
 
    //$valid_chars = array('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z');
    //$valid_chars = range('a', 'z');
    $english_low    = range('a', 'z');
    $english_cap    = range('A', 'Z');
    $greek_low      = array('α', 'β');
    $greek_cap      = array('Α', 'Β');
    $valid_chars    = array_merge($english_low, $english_cap, $greek_low, $greek_cap);
 
    $errors = 0;
 
    for($i = 0; $i < mb_strlen($string, 'UTF8'); $i++ ) {
        $char = mb_substr($string, $i, 1, 'UTF8');
        echo $char . PHP_EOL;
        if (!in_array($char, $valid_chars)) { $errors++; }
    }
 
    echo "\n\r".$errors."\n\r";
 
}

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