预定义常量

下列常量作为 PHP 核心的一部分总是可用的。

**openlog() Options**
Constant	Description
`LOG_CONS`	if there is an error while sending data to the system logger, write directly to the system console
`LOG_NDELAY`	open the connection to the logger immediately
`LOG_ODELAY`	(default) delay opening the connection until the first message is logged
`LOG_NOWAIT`
`LOG_PERROR`	print log message also to standard error
`LOG_PID`	include PID with each message

**openlog() Facilities**
Constant	Description
`LOG_AUTH`	security/authorization messages (use `LOG_AUTHPRIV` instead in systems where that constant is defined)
`LOG_AUTHPRIV`	security/authorization messages (private)
`LOG_CRON`	clock daemon (cron and at)
`LOG_DAEMON`	other system daemons
`LOG_KERN`	kernel messages
`LOG_LOCAL0 ... LOG_LOCAL7`	reserved for local use, these are not available in Windows
`LOG_LPR`	line printer subsystem
`LOG_MAIL`	mail subsystem
`LOG_NEWS`	USENET news subsystem
`LOG_SYSLOG`	messages generated internally by syslogd
`LOG_USER`	generic user-level messages
`LOG_UUCP`	UUCP subsystem

**syslog() Priorities (in descending order)**
Constant	Description
`LOG_EMERG`	system is unusable
`LOG_ALERT`	action must be taken immediately
`LOG_CRIT`	critical conditions
`LOG_ERR`	error conditions
`LOG_WARNING`	warning conditions
`LOG_NOTICE`	normal, but significant, condition
`LOG_INFO`	informational message
`LOG_DEBUG`	debug-level message

**dns_get_record() Options**
Constant	Description
`DNS_A`	IPv4 Address Resource
`DNS_CAA`	Certification Authority Authorization Resource (available as of PHP 7.0.16 and 7.1.2)
`DNS_MX`	Mail Exchanger Resource
`DNS_CNAME`	Alias (Canonical Name) Resource
`DNS_NS`	Authoritative Name Server Resource
`DNS_PTR`	Pointer Resource
`DNS_HINFO`	Host Info Resource (See IANA's » Operating System Names for the meaning of these values)
`DNS_SOA`	Start of Authority Resource
`DNS_TXT`	Text Resource
`DNS_ANY`	Any Resource Record. On most systems this returns all resource records, however it should not be counted upon for critical uses. Try `DNS_ALL` instead.
`DNS_AAAA`	IPv6 Address Resource
`DNS_ALL`	Iteratively query the name server for each available record type.