Caching
The caching system has been rewritten.
Documentation
Files Affected
Configuration
variable_get() has been removed.
Documentation
Files Affected
Entity API
user_load() has been removed.
Documentation
Files Affected
user_save() has been removed.
Documentation
Files Affected
Form API
form_get_errors() is now a method of FormStateInterface.
Documentation
Files Affected
form_state_values_clean() is now a method of FormStateInterface.
Documentation
Files Affected
Info File
Module info files' core key must have a value of 8.x.
Documentation
Files Affected
Info files must contain a type key.
Documentation
Files Affected
Many common dependencies have moved into core.
Documentation
Files Affected
Module info files' configure key must be a route name, not a path.
Documentation
Files Affected
Menu/Routing
hook_menu() has been removed from Drupal 8.
Documentation
Files Affected
Miscellaneous
Many common functions, shared variables, and constants have been renamed.
Documentation
Automated web tests must be in a PSR-4 namespace, and unit tests must be converted to PHPUnit.
Documentation
Rendering
drupal_add_css() has been removed.
Documentation
Files Affected
drupal_add_js() has been removed.
Documentation
Files Affected
System
drupal_goto() has been removed.
Documentation
Files Affected
drupal_http_request() has been replaced by Guzzle.
Documentation
Files Affected
drupal_json_decode() has moved into the Serialization component.
Documentation
Files Affected
l() has been removed.
Documentation
Files Affected
module_invoke_all() is now a method of the module_handler service.
Documentation
Files Affected
url() has been removed.
Documentation
Files Affected
watchdog() has been removed.
Documentation
Files Affected
Theme System
theme() has been renamed to _theme(), and should never be called directly.
Documentation
Files Affected
Utilities
drupal_strlen() has been moved into the Unicode utility class.
Documentation
Files Affected
drupal_strtolower() has been moved into the Unicode utility class.
Documentation
Files Affected
drupal_substr() has been moved into the Unicode utility class.
Documentation
Files Affected