/wp-includes/compat.php

<?php
/**
 * WordPress implementation for PHP functions either missing from older PHP versions or not included by default.
 *
 * @package PHP
 * @access private
 */

// If gettext isn't available
if ( !function_exists('_') ) {
	function _($string) {
		return $string;
	}
}

/**
 * Returns whether PCRE/u (PCRE_UTF8 modifier) is available for use.
 *
 * @ignore
 * @since 4.2.2
 * @access private
 *
 * @staticvar string $utf8_pcre
 *
 * @param bool $set - Used for testing only
 *             null   : default - get PCRE/u capability
 *             false  : Used for testing - return false for future calls to this function
 *             'reset': Used for testing - restore default behavior of this function
 */
function _wp_can_use_pcre_u( $set = null ) {
	static $utf8_pcre = 'reset';

	if ( null !== $set ) {
		$utf8_pcre = $set;
	}

	if ( 'reset' === $utf8_pcre ) {
		$utf8_pcre = @preg_match( '/^./u', 'a' );
	}

	return $utf8_pcre;
}

if ( ! function_exists( 'mb_substr' ) ) :
	function mb_substr( $str, $start, $length = null, $encoding = null ) {
		return _mb_substr( $str, $start, $length, $encoding );
	}
endif;

/*
 * Only understands UTF-8 and 8bit.  All other character sets will be treated as 8bit.
 * For $encoding === UTF-8, the $str input is expected to be a valid UTF-8 byte sequence.
 * The behavior of this function for invalid inputs is undefined.
 */
function _mb_substr( $str, $start, $length = null, $encoding = null ) {
	if ( null === $encoding ) {
		$encoding = get_option( 'blog_charset' );
	}

	// The solution below works only for UTF-8,
	// so in case of a different charset just use built-in substr()
	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) ) ) {
		return is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );
	}

	if ( _wp_can_use_pcre_u() ) {
		// Use the regex unicode support to separate the UTF-8 characters into an array
		preg_match_all( '/./us', $str, $match );
		$chars = is_null( $length ) ? array_slice( $match[0], $start ) : array_slice( $match[0], $start, $length );
		return implode( '', $chars );
	}

	$regex = '/(
		  [\x00-\x7F]                  # single-byte sequences   0xxxxxxx
		| [\xC2-\xDF][\x80-\xBF]       # double-byte sequences   110xxxxx 10xxxxxx
		| \xE0[\xA0-\xBF][\x80-\xBF]   # triple-byte sequences   1110xxxx 10xxxxxx * 2
		| [\xE1-\xEC][\x80-\xBF]{2}
		| \xED[\x80-\x9F][\x80-\xBF]
		| [\xEE-\xEF][\x80-\xBF]{2}
		| \xF0[\x90-\xBF][\x80-\xBF]{2} # four-byte sequences   11110xxx 10xxxxxx * 3
		| [\xF1-\xF3][\x80-\xBF]{3}
		| \xF4[\x80-\x8F][\x80-\xBF]{2}
	)/x';

	$chars = array( '' ); // Start with 1 element instead of 0 since the first thing we do is pop
	do {
		// We had some string left over from the last round, but we counted it in that last round.
		array_pop( $chars );

		// Split by UTF-8 character, limit to 1000 characters (last array element will contain the rest of the string)
		$pieces = preg_split( $regex, $str, 1000, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY );

		$chars = array_merge( $chars, $pieces );
	} while ( count( $pieces ) > 1 && $str = array_pop( $pieces ) ); // If there's anything left over, repeat the loop.

	return join( '', array_slice( $chars, $start, $length ) );
}

if ( ! function_exists( 'mb_strlen' ) ) :
	function mb_strlen( $str, $encoding = null ) {
		return _mb_strlen( $str, $encoding );
	}
endif;

/*
 * Only understands UTF-8 and 8bit.  All other character sets will be treated as 8bit.
 * For $encoding === UTF-8, the $str input is expected to be a valid UTF-8 byte sequence.
 * The behavior of this function for invalid inputs is undefined.
 */
function _mb_strlen( $str, $encoding = null ) {
	if ( null === $encoding ) {
		$encoding = get_option( 'blog_charset' );
	}

	// The solution below works only for UTF-8,
	// so in case of a different charset just use built-in strlen()
	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) ) ) {
		return strlen( $str );
	}

	if ( _wp_can_use_pcre_u() ) {
		// Use the regex unicode support to separate the UTF-8 characters into an array
		preg_match_all( '/./us', $str, $match );
		return count( $match[0] );
	}

	$regex = '/(?:
		  [\x00-\x7F]                  # single-byte sequences   0xxxxxxx
		| [\xC2-\xDF][\x80-\xBF]       # double-byte sequences   110xxxxx 10xxxxxx
		| \xE0[\xA0-\xBF][\x80-\xBF]   # triple-byte sequences   1110xxxx 10xxxxxx * 2
		| [\xE1-\xEC][\x80-\xBF]{2}
		| \xED[\x80-\x9F][\x80-\xBF]
		| [\xEE-\xEF][\x80-\xBF]{2}
		| \xF0[\x90-\xBF][\x80-\xBF]{2} # four-byte sequences   11110xxx 10xxxxxx * 3
		| [\xF1-\xF3][\x80-\xBF]{3}
		| \xF4[\x80-\x8F][\x80-\xBF]{2}
	)/x';

	$count = 1; // Start at 1 instead of 0 since the first thing we do is decrement
	do {
		// We had some string left over from the last round, but we counted it in that last round.
		$count--;

		// Split by UTF-8 character, limit to 1000 characters (last array element will contain the rest of the string)
		$pieces = preg_split( $regex, $str, 1000 );

		// Increment
		$count += count( $pieces );
	} while ( $str = array_pop( $pieces ) ); // If there's anything left over, repeat the loop.

	// Fencepost: preg_split() always returns one extra item in the array
	return --$count;
}

if ( !function_exists('hash_hmac') ):
function hash_hmac($algo, $data, $key, $raw_output = false) {
	return _hash_hmac($algo, $data, $key, $raw_output);
}
endif;

function _hash_hmac($algo, $data, $key, $raw_output = false) {
	$packs = array('md5' => 'H32', 'sha1' => 'H40');

	if ( !isset($packs[$algo]) )
		return false;

	$pack = $packs[$algo];

	if (strlen($key) > 64)
		$key = pack($pack, $algo($key));

	$key = str_pad($key, 64, chr(0));

	$ipad = (substr($key, 0, 64) ^ str_repeat(chr(0x36), 64));
	$opad = (substr($key, 0, 64) ^ str_repeat(chr(0x5C), 64));

	$hmac = $algo($opad . pack($pack, $algo($ipad . $data)));

	if ( $raw_output )
		return pack( $pack, $hmac );
	return $hmac;
}

if ( !function_exists('json_encode') ) {
	function json_encode( $string ) {
		global $wp_json;

		if ( ! ( $wp_json instanceof Services_JSON ) ) {
			require_once( ABSPATH . WPINC . '/class-json.php' );
			$wp_json = new Services_JSON();
		}

		return $wp_json->encodeUnsafe( $string );
	}
}

if ( !function_exists('json_decode') ) {
	/**
	 * @global Services_JSON $wp_json
	 * @param string $string
	 * @param bool   $assoc_array
	 * @return object|array
	 */
	function json_decode( $string, $assoc_array = false ) {
		global $wp_json;

		if ( ! ($wp_json instanceof Services_JSON ) ) {
			require_once( ABSPATH . WPINC . '/class-json.php' );
			$wp_json = new Services_JSON();
		}

		$res = $wp_json->decode( $string );
		if ( $assoc_array )
			$res = _json_decode_object_helper( $res );
		return $res;
	}

	/**
	 * @param object $data
	 * @return array
	 */
	function _json_decode_object_helper($data) {
		if ( is_object($data) )
			$data = get_object_vars($data);
		return is_array($data) ? array_map(__FUNCTION__, $data) : $data;
	}
}

if ( ! function_exists( 'hash_equals' ) ) :
/**
 * Compare two strings in constant time.
 *
 * This function was added in PHP 5.6.
 * It can leak the length of a string.
 *
 * @since 3.9.2
 *
 * @param string $a Expected string.
 * @param string $b Actual string.
 * @return bool Whether strings are equal.
 */
function hash_equals( $a, $b ) {
	$a_length = strlen( $a );
	if ( $a_length !== strlen( $b ) ) {
		return false;
	}
	$result = 0;

	// Do not attempt to "optimize" this.
	for ( $i = 0; $i < $a_length; $i++ ) {
		$result |= ord( $a[ $i ] ) ^ ord( $b[ $i ] );
	}

	return $result === 0;
}
endif;

// JSON_PRETTY_PRINT was introduced in PHP 5.4
// Defined here to prevent a notice when using it with wp_json_encode()
if ( ! defined( 'JSON_PRETTY_PRINT' ) ) {
	define( 'JSON_PRETTY_PRINT', 128 );
}

if ( ! function_exists( 'json_last_error_msg' ) ) :
	/**
	 * Retrieves the error string of the last json_encode() or json_decode() call.
	 *
	 * @since 4.4.0
	 *
	 * @internal This is a compatibility function for PHP <5.5
	 *
	 * @return bool|string Returns the error message on success, "No Error" if no error has occurred,
	 *                     or false on failure.
	 */
	function json_last_error_msg() {
		// See https://core.trac.wordpress.org/ticket/27799.
		if ( ! function_exists( 'json_last_error' ) ) {
			return false;
		}

		$last_error_code = json_last_error();

		// Just in case JSON_ERROR_NONE is not defined.
		$error_code_none = defined( 'JSON_ERROR_NONE' ) ? JSON_ERROR_NONE : 0;

		switch ( true ) {
			case $last_error_code === $error_code_none:
				return 'No error';

			case defined( 'JSON_ERROR_DEPTH' ) && JSON_ERROR_DEPTH === $last_error_code:
				return 'Maximum stack depth exceeded';

			case defined( 'JSON_ERROR_STATE_MISMATCH' ) && JSON_ERROR_STATE_MISMATCH === $last_error_code:
				return 'State mismatch (invalid or malformed JSON)';

			case defined( 'JSON_ERROR_CTRL_CHAR' ) && JSON_ERROR_CTRL_CHAR === $last_error_code:
				return 'Control character error, possibly incorrectly encoded';

			case defined( 'JSON_ERROR_SYNTAX' ) && JSON_ERROR_SYNTAX === $last_error_code:
				return 'Syntax error';

			case defined( 'JSON_ERROR_UTF8' ) && JSON_ERROR_UTF8 === $last_error_code:
				return 'Malformed UTF-8 characters, possibly incorrectly encoded';

			case defined( 'JSON_ERROR_RECURSION' ) && JSON_ERROR_RECURSION === $last_error_code:
				return 'Recursion detected';

			case defined( 'JSON_ERROR_INF_OR_NAN' ) && JSON_ERROR_INF_OR_NAN === $last_error_code:
				return 'Inf and NaN cannot be JSON encoded';

			case defined( 'JSON_ERROR_UNSUPPORTED_TYPE' ) && JSON_ERROR_UNSUPPORTED_TYPE === $last_error_code:
				return 'Type is not supported';

			default:
				return 'An unknown error occurred';
		}
	}
endif;

if ( ! interface_exists( 'JsonSerializable' ) ) {
	define( 'WP_JSON_SERIALIZE_COMPATIBLE', true );
	/**
	 * JsonSerializable interface.
	 *
	 * Compatibility shim for PHP <5.4
	 *
     * @link http://php.net/jsonserializable
	 *
	 * @since 4.4.0
	 */
	interface JsonSerializable {
		public function jsonSerialize();
	}
}

// random_int was introduced in PHP 7.0
if ( ! function_exists( 'random_int' ) ) {
	require ABSPATH . WPINC . '/random_compat/random.php';
}