PHP摩尔斯电码转换器

我正在用 PHP编写一个基本的莫尔斯代码转换器,它可以接受一个字符串并将其转换为莫尔斯代码.它使用了一个关联数组,一个foreach循环和一个for循环.它有效,除了某些原因它在每个转换后的字符后输出等效于’0’的莫尔斯码.我无法弄清楚0的来源.如果我从关联数组中删除0,没有问题,但我希望能够转换数字.如果有人能够给我一些反馈,那将非常感激.

这是代码：

<?php
$string = "dog";
$string_lower = strtolower($string);
$assoc_array = array(
    "a"=>".-","b"=>"-...","c"=>"-.-.","d"=>"-..","e"=>".","f"=>"..-.","g"=>"--.","h"=>"....","i"=>"..","j"=>".---","k"=>"-.-","l"=>".-..","m"=>"--","n"=>"-.","o"=>"---","p"=>".--.","q"=>"--.-","r"=>".-.","s"=>"...","t"=>"-","u"=>"..-","v"=>"...-","w"=>".--","x"=>"-..-","y"=>"-.--","z"=>"--..","0"=>"-----","1"=>".----","2"=>"..---","3"=>"...--","4"=>"....-","5"=>".....","6"=>"-....","7"=>"--...","8"=>"---..","9"=>"----.","."=>".-.-.-",","=>"--..--","?"=>"..--..","/"=>"-..-."," "=>" ");
    for($i=0;$i<strlen($string_lower);$i++){
        foreach($assoc_array as $letter => $code){
            if($letter == $string_lower[$i]){
                echo "$code<br/>";
            }
        }
    }
?>

/*Rest of your code above*/
for($i=0;$i<strlen($string_lower);$i++){
    echo (isset($assoc_array[$string_lower[$i]])) ? $assoc_array[$string_lower[$i]] . '<br />' : 'ERROR';       
}