将PHP字符串从24个字符压缩（缩短）到20个字符

I'm currently writing a PHP application that utilizes the Authorize.net api. This api requires that a customer's unique id value be less than 20 characters. I'm currently storing this unique customer id in Mongo as a MongoId object (24 characters).

Is there a way to convert a 24 character string to 20 so that it can meet the API requirements ?

From what I see on your referenced page, the 24-characters are hexadecimal. If customer-id may be alphanumerical you can use base_convert to shorten the number. Unfortunately the full number is > 32bit so you need to covert it in parts to make it work:

// Pad with 0's to make sure you have 24 chars
$padded = str_repeat('0', 24 - strlen($mongoId)) . $mongoId;
$leastSignificant = base_convert(substr($padded, 14, 10), 16, 32); // will be 8 chars most
$middleSignificant = base_convert(substr($padded, 4, 10), 16, 32); // will be 8 chars most
$highSignificant = base_convert(substr($padded, 0, 4), 16, 32); // will be 4 chars most

// Concatenate, and make sure everything is correctly padded
$result = str_repeat('0', 4 - strlen($highSignificant)) . $highSignificant .
          str_repeat('0', 8 - strlen($middleSignificant )) . $middleSignificant .
          str_repeat('0', 8 - strlen($leastSignificant )) . $leastSignificant;
echo strlen($result); // Will echo 20

// Reverse the algoritm to retrieve the mongoId for a given customerId