Decode URL from Base64 format
The term Base64 is coming from a certain MIME content transfer encoding. Basically, Base64 is a collection of related encoding designs which represent the binary information in ASCII format by converting it into a base64 representation.
Base64 encoding schemes are generally used when there is a need to encode binary information that needs to be stored and transferred over media that are developed to deal with textual information. This guarantees that the data stays unchanged without modification during transfer. Base64 is generally used in a number of applications including electronic mail via MIME, and keeping complex information in XML.
The specific set of characters chosen for the 64 characters needed for the base can vary among implementations. The common concept is to select a set of 64 characters that is both part of a subset typical to most encodings. This mixture leaves the data impossible to be altered in transportation thru information systems, such as electronic mail, that were typically not 8-bit clean. The Base64 implementation in MIME uses a-z, A-Z and 0-9 for the first 62 values. Other Base64 variations share the same property but they use different symbols in the last two values.
Base64 encoding can be helpful when fairly lengthy identifying information is used in an HTTP environment. For example, a database persistence framework for Java objects might use Base64 encoding to encode a relatively large unique id (generally 128-bit UUIDs) into a string for use as an HTTP parameter in HTTP forms or HTTP GET URLs. Also, many applications need to encode binary data in a way that is convenient for inclusion in URLs, including in hidden web form fields, and Base64 is a convenient encoding to render them in a compact way.
Using standard Base64 in URL requires encoding of '+', '/' and '=' characters into special percent-encoded hexadecimal sequences ('+' becomes '%2B', '/' becomes '%2F' and '=' becomes '%3D'), which makes the string unnecessarily longer. For this reason, modified Base64 for URL variants exist, where the '+' and '/' characters of standard Base64 are respectively replaced by '-' and '_', so that using URL encoders/decoders is no longer necessary and have no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general.
Some variants allow or require omitting the padding '=' signs to avoid them being confused with field separators, or require that any such padding be percent-encoded. Some libraries will encode '=' to '.'.
Base64 index table:
Base 64 URL Decode PHP Example
$result = base64_decode(str_replace(['-','_'], ['+','/'], $base64));
Source: Base64 on Wikipedia