A file that contains a ton of cool constants for PHP projects. Geared for the CodeIgniter framework, but it is not required.

constants.php

<?php defined('BASEPATH') || exit('No direct script access allowed');
/**
 * --------------------------------------------------------------------------
 *  Application Specific Constants
 * --------------------------------------------------------------------------
 *
 * The following constants are application specific. They cover items such as
 * writable path values, HTTP status codes and more!
 *
 * @package  Constants
 * @author   Jason Napolitano <jnapolitanoit@gmail.com>
 * @updated  10.18.2020
 *
 * @license  2-clause BSD
 * @version  1.3.0 [major.minor.patch]
 * @since    1.0.0 [major.minor.patch]
 *
 */
// ----------------------------------------------------------------------------
// DIRECTORY_SEPARATOR Aliased as _DS_
define('_DS_', str_replace(DIRECTORY_SEPARATOR, '/', '\\'));

// Application PSR-4 Namespace and Path
define('APPPATH', _DIR__ . '/path/to/app-folder');

// All files live inside of App\*
define('APP_NAMESPACE', 'App');

// ----------------------------------------------------------------------------
// Writable Directory Constants
define('WRITEPATH', APPPATH   . 'writable'   . _DS_);
define('CACHEPATH', WRITEPATH . 'cache'      . _DS_);
define('SESSPATH',  WRITEPATH . 'sessions'   . _DS_);
define('LOGPATH',   WRITEPATH . 'logs'       . _DS_);

/**
 * --------------------------------------------------------------------------
 *  Client/Server HTTP status code constants
 * --------------------------------------------------------------------------
 *
 *  These are the constants that you may use anywhere in the application for
 *  allowing easier, quick access to status codes.
 *
 *  NOTE: All Credit fort this list goes to Phil Sturgeon and Chris Kacerguis
 *        since these were taken from the rest controller library/
 *  @link https://github.com/chriskacerguis/codeigniter-restserver/
 */

// ----------------------------------------------------------------------------
// HTTP Status Codes
define('HTTP_CONTINUE',            100);
define('HTTP_SWITCHING_PROTOCOLS', 101);
define('HTTP_PROCESSING',          102); // RFC2518

// ----------------------------------------------------------------------------
// The request has succeeded
define('HTTP_OK', 200);

// ----------------------------------------------------------------------------
// The server successfully created a new resource
define('HTTP_CREATED',                       201);
define('HTTP_ACCEPTED',                      202);
define('HTTP_NON_AUTHORITATIVE_INFORMATION', 203);

// ----------------------------------------------------------------------------
// The server successfully processed the request, though no content is returned
define('HTTP_NO_CONTENT',       204);
define('HTTP_RESET_CONTENT',    205);
define('HTTP_PARTIAL_CONTENT',  206);
define('HTTP_MULTI_STATUS',     207); // RFC4918
define('HTTP_ALREADY_REPORTED', 208); // RFC5842
define('HTTP_IM_USED',          226); // RFC3229

// ----------------------------------------------------------------------------
// Redirection
define('HTTP_MULTIPLE_CHOICES',  300);
define('HTTP_MOVED_PERMANENTLY', 301);
define('HTTP_FOUND',             302);
define('HTTP_SEE_OTHER',         303);

// ----------------------------------------------------------------------------
// The resource has not been modified since the last request
define('HTTP_NOT_MODIFIED',         304);
define('HTTP_USE_PROXY',            305);
define('HTTP_RESERVED',             306);
define('HTTP_TEMPORARY_REDIRECT',   307);
define('HTTP_PERMANENTLY_REDIRECT', 308);   // RFC7238

// ----------------------------------------------------------------------------
// The request cannot be fulfilled due to multiple errors
define('HTTP_BAD_REQUEST', 400);

// ----------------------------------------------------------------------------
// The user is unauthorized to access the requested resource
define('HTTP_UNAUTHORIZED',     401);
define('HTTP_PAYMENT_REQUIRED', 402);

// ----------------------------------------------------------------------------
// The requested resource is unavailable at this present time
define('HTTP_FORBIDDEN', 403);

// ----------------------------------------------------------------------------
// The requested resource could not be found
define('HTTP_NOT_FOUND', 404);

// ----------------------------------------------------------------------------
// The request method is not supported by the following resource
define('HTTP_METHOD_NOT_ALLOWED', 405);

// ----------------------------------------------------------------------------
// The request was not acceptable
define('HTTP_NOT_ACCEPTABLE',                406);
define('HTTP_PROXY_AUTHENTICATION_REQUIRED', 407);
define('HTTP_REQUEST_TIMEOUT',               408);

// ----------------------------------------------------------------------------
// The request could not be completed due to a conflict with the current state
// of the resource
define('HTTP_CONFLICT',                                                  409);
define('HTTP_GONE',                                                      410);
define('HTTP_LENGTH_REQUIRED',                                           411);
define('HTTP_PRECONDITION_FAILED',                                       412);
define('HTTP_REQUEST_ENTITY_TOO_LARGE',                                  413);
define('HTTP_REQUEST_URI_TOO_LONG',                                      414);
define('HTTP_UNSUPPORTED_MEDIA_TYPE',                                    415);
define('HTTP_REQUESTED_RANGE_NOT_SATISFIABLE',                           416);
define('HTTP_EXPECTATION_FAILED',                                        417);
define('HTTP_I_AM_A_TEAPOT',                                             418); // RFC2324
define('HTTP_UNPROCESSABLE_ENTITY',                                      422); // RFC4918
define('HTTP_LOCKED',                                                    423); // RFC4918
define('HTTP_FAILED_DEPENDENCY',                                         424); // RFC4918
define('HTTP_RESERVED_FOR_WEBDAV_ADVANCED_COLLECTIONS_EXPIRED_PROPOSAL', 425); // RFC2817
define('HTTP_UPGRADE_REQUIRED',                                          426); // RFC2817
define('HTTP_PRECONDITION_REQUIRED',                                     428); // RFC6585
define('HTTP_TOO_MANY_REQUESTS',                                         429); // RFC6585
define('HTTP_REQUEST_HEADER_FIELDS_TOO_LARGE',                           431); // RFC6585

// ----------------------------------------------------------------------------
// The server encountered an unexpected error
// Note: This is a generic error message when no specific message
// is suitable
define('HTTP_INTERNAL_SERVER_ERROR', 500);

// ----------------------------------------------------------------------------
// The server does not recognise the request method
define('HTTP_NOT_IMPLEMENTED',                      501);
define('HTTP_BAD_GATEWAY',                          502);
define('HTTP_SERVICE_UNAVAILABLE',                  503);
define('HTTP_GATEWAY_TIMEOUT',                      504);
define('HTTP_VERSION_NOT_SUPPORTED',                505);
define('HTTP_VARIANT_ALSO_NEGOTIATES_EXPERIMENTAL', 506); // RFC2295
define('HTTP_INSUFFICIENT_STORAGE',                 507); // RFC4918
define('HTTP_LOOP_DETECTED',                        508); // RFC5842
define('HTTP_NOT_EXTENDED',                         510); // RFC2774
define('HTTP_NETWORK_AUTHENTICATION_REQUIRED',      511);

/*
 * --------------------------------------------------------------------------
 *  Display Debug backtrace
 * --------------------------------------------------------------------------
 *
 *  If set to TRUE, a backtrace will be displayed along with php errors. If
 *  error_reporting is disabled, the backtrace will not display, regardless
 *  of this setting
 *
 */
defined('SHOW_DEBUG_BACKTRACE') || define('SHOW_DEBUG_BACKTRACE', TRUE);

/*
 * --------------------------------------------------------------------------
 *  File and Directory Modes
 * --------------------------------------------------------------------------
 *
 *  These prefs are used when checking and setting modes when working
 *  with the file system.  The defaults are fine on servers with proper
 *  security, but you may wish (or even need) to change the values in
 *  certain environments (Apache running a separate process for each
 *  user, PHP under CGI with Apache suEXEC, etc.).  Octal values should
 *  always be used to set the mode correctly.
 *
 */
defined('FILE_READ_MODE')  || define('FILE_READ_MODE', 0644);
defined('FILE_WRITE_MODE') || define('FILE_WRITE_MODE', 0666);
defined('DIR_READ_MODE')   || define('DIR_READ_MODE', 0755);
defined('DIR_WRITE_MODE')  || define('DIR_WRITE_MODE', 0755);

/*
 * --------------------------------------------------------------------------
 *  File Stream Modes
 * --------------------------------------------------------------------------
 *
 *  These modes are used when working with fopen()/popen()
 *
 */
defined('FOPEN_READ')                          || define('FOPEN_READ', 'rb');
defined('FOPEN_READ_WRITE')                    || define('FOPEN_READ_WRITE', 'r+b');
defined('FOPEN_WRITE_CREATE_DESTRUCTIVE')      || define('FOPEN_WRITE_CREATE_DESTRUCTIVE', 'wb'); // truncates existing file data, use with care
defined('FOPEN_READ_WRITE_CREATE_DESTRUCTIVE') || define('FOPEN_READ_WRITE_CREATE_DESTRUCTIVE', 'w+b'); // truncates existing file data, use with care
defined('FOPEN_WRITE_CREATE')                  || define('FOPEN_WRITE_CREATE', 'ab');
defined('FOPEN_READ_WRITE_CREATE')             || define('FOPEN_READ_WRITE_CREATE', 'a+b');
defined('FOPEN_WRITE_CREATE_STRICT')           || define('FOPEN_WRITE_CREATE_STRICT', 'xb');
defined('FOPEN_READ_WRITE_CREATE_STRICT')      || define('FOPEN_READ_WRITE_CREATE_STRICT', 'x+b');

/*
 * --------------------------------------------------------------------------
 *  Exit Status Codes
 * --------------------------------------------------------------------------
 *
 *  Used to indicate the conditions under which the script is exit()ing.
 *  While there is no universal standard for error codes, there are some
 *  broad conventions.  Three such conventions are mentioned below, for
 *  those who wish to make use of them.  The CodeIgniter defaults were
 *  chosen for the least overlap with these conventions, while still
 *  leaving room for others to be defined in future versions and user
 *  applications.
 *
 *  The three main conventions used for determining exit status codes
 *  are as follows:
 *
 *     Standard C/C++ Library (stdlibc):
 *        http://www.gnu.org/software/libc/manual/html_node/Exit-Status.html
 *        (This link also contains other GNU-specific conventions)
 *     BSD sysexits.h:
 *        http://www.gsp.com/cgi-bin/man.cgi?section=3&topic=sysexits
 *     Bash scripting:
 *        http://tldp.org/LDP/abs/html/exitcodes.html
 *
 */
defined('EXIT_SUCCESS')        || define('EXIT_SUCCESS', 0);        // no errors
defined('EXIT_ERROR')          || define('EXIT_ERROR', 1);          // generic error
defined('EXIT_CONFIG')         || define('EXIT_CONFIG', 3);         // configuration error
defined('EXIT_UNKNOWN_FILE')   || define('EXIT_UNKNOWN_FILE', 4);   // file not found
defined('EXIT_UNKNOWN_CLASS')  || define('EXIT_UNKNOWN_CLASS', 5);  // unknown class
defined('EXIT_UNKNOWN_METHOD') || define('EXIT_UNKNOWN_METHOD', 6); // unknown class member
defined('EXIT_USER_INPUT')     || define('EXIT_USER_INPUT', 7);     // invalid user input
defined('EXIT_DATABASE')       || define('EXIT_DATABASE', 8);       // database error
defined('EXIT__AUTO_MIN')      || define('EXIT__AUTO_MIN', 9);      // lowest automatically-assigned error code
defined('EXIT__AUTO_MAX')      || define('EXIT__AUTO_MAX', 125);    // highest automatically-assigned error code