Validation

The parser builds the model without judging it; the validator checks RFC 5545 semantics:

use om\ICal\Validation\Severity;
use om\ICal\Validation\Validator;

foreach ((new Validator())->validate($calendar) as $issue) {
  echo $issue;  // ERROR VEVENT.DURATION (line 12): DTEND and DURATION must not occur together. [component.end-and-duration]
}
(new Validator())->assertValid($calendar); // ValidationException with the first error

An Issue has a Severity (Info, Warning, Error), a code, a message, the component, the property, the line and the uid.

Code	Severity	Rule
component.missing-property	Error	PRODID and VERSION of VCALENDAR, UID and DTSTAMP of VEVENT, VTODO, VJOURNAL and VFREEBUSY, TZID of VTIMEZONE, DTSTART, TZOFFSETFROM and TZOFFSETTO of observances, ACTION and TRIGGER of VALARM, UID of VLOCATION (RFC 9073)
component.duplicate-property	Error	a property that may occur only once occurs again
component.end-and-duration	Error	DTEND (DUE) together with DURATION
component.end-type	Error	DTEND (DUE) has another value type than DTSTART
component.end-before-start	Error	DTEND (DUE) is before DTSTART
component.duration-without-start	Error	DURATION without DTSTART
recurrence.count-and-until	Error	COUNT together with UNTIL
recurrence.until-type	Error	UNTIL is not a date for a date DTSTART, not UTC for a UTC or zoned DTSTART, not local for a floating DTSTART
recurrence.without-start	Error	RRULE or RDATE without DTSTART
recurrence.multiple-rrule	Warning	more than one RRULE
recurrence.skip-without-rscale	Error	SKIP without RSCALE (RFC 7529)
recurrence.unsupported-rscale	Warning	an RRULE of another calendar system than GREGORIAN or with a leap month, it is not expanded
value.invalid	Error	a value that does not match its type
value.not-utc	Warning	ACKNOWLEDGED of VALARM (RFC 9074) that is not a UTC time
timezone.not-defined	Warning	a TZID without a VTIMEZONE definition
timezone.unresolved	Warning	a TZID that cannot be resolved
timezone.no-observance	Error	a VTIMEZONE without STANDARD and DAYLIGHT
alarm.outside-item	Error	a VALARM directly in VCALENDAR