urlencode() / urldecode() / base64_encode() Since: PHP 4（2000）

// URL-encodes a string. Spaces are converted to '+'.
urlencode($string);

// URL-decodes a string.
urldecode($string);

// URL-encodes a string per RFC 3986. Spaces are converted to '%20'.
rawurlencode($string);

// URL-decodes a string per RFC 3986.
rawurldecode($string);

// Base64-encodes a string.
base64_encode($string);

// Base64-decodes a string.
base64_decode($string, $strict);

Returns the encoded or decoded string. base64_decode() returns false if decoding fails.

<?php
// URL-encode a Japanese string.
echo urlencode("東京都"); // Outputs '%E6%9D%B1%E4%BA%AC%E9%83%BD'.

// Example of building a query parameter with URL encoding.
$keyword = "PHP tutorial";
$url = "https://example.com/search?q=" . urlencode($keyword);
echo $url; // Outputs 'https://example.com/search?q=PHP+tutorial'.

// rawurlencode() converts spaces to '%20'.
echo rawurlencode("PHP tutorial"); // Outputs the string with spaces as '%20'.

// Decode a URL-encoded string back to the original.
echo urldecode("%E6%9D%B1%E4%BA%AC%E9%83%BD"); // Outputs '東京都'.

// The difference between urlencode() and rawurlencode().
echo urlencode("hello world"); // Outputs 'hello+world'.
echo rawurlencode("hello world"); // Outputs 'hello%20world'.

// Base64 encoding.
echo base64_encode("Hello PHP"); // Outputs 'SGVsbG8gUEhQ'.

// Decode Base64 back to the original string.
echo base64_decode("SGVsbG8gUEhQ"); // Outputs 'Hello PHP'.

// Example of encoding data for an email attachment or API request.
$data = json_encode(["user" => "John", "action" => "purchase"]);
$encoded = base64_encode($data);
echo $encoded; // Outputs the data as a safe Base64 string.

// Decode back to the original data.
$decoded = json_decode(base64_decode($encoded), true);
echo $decoded["user"]; // Outputs 'John'.

// Example of safely building multiple query parameters at once.
$params = [
	"name" => "John Smith",
	"city" => "New York"
];
echo http_build_query($params); // Automatically encodes all parameters.

urlencode() encodes values for use in URL query parameters, allowing special characters such as Japanese text and spaces to be safely included in a URL. urlencode() converts spaces to '+', while rawurlencode() converts them to '%20'. In general, use urlencode() for query parameter values and rawurlencode() for the path portion of a URL.

When you need to encode multiple parameters at once, http_build_query() is the convenient choice. Simply pass an associative array and it automatically encodes all keys and values into a query string.

base64_encode() converts binary data to an ASCII string, and is used for email attachments and sending or receiving data via APIs. Base64 is an encoding scheme, not encryption — do not use it to protect sensitive data. To escape HTML output, use htmlspecialchars().