PHP reads the text file and outputs the most used strings and corresponding times in the line line by line.

$myfile = fopen("test.txt", "r");
while(!feof($myfile)) {
    $line_str = fgets($myfile);
    $str_arr = count_chars($line_str, 1);
    arsort($str_arr);
    print_r(chr(key($str_arr)).' is '.current($str_arr).PHP_EOL);
}
fclose($myfile);