When PHP parses a file, it simply passes the text of
the file through until it encounters one of the special
tags which tell it to start interpreting the text as PHP
code. The parser then executes all the code it finds, up
until it runs into a PHP closing tag, which tells the
parser to just start passing the text through again. This
is the mechanism which allows you to embed PHP code inside
HTML: everything outside the PHP tags is left utterly
alone, while everything inside is parsed as code.
There are four sets of tags which can be used to
denote blocks of PHP code. Of these, only two ( ?php. .
.? and script language="php" . .
. /script ) are always available; the others can be
turned on or off from the php.ini
configuration file. While the short-form tags and ASP-style
tags may be convenient, they are not as portable as the
longer versions. Also, if you intend to embed PHP code in
XML or XHTML, you will need to use the ?php. . .?
form to conform to the XML.
The tags supported by PHP are:
The first way, ?php. . .? , is the preferred
method, as it allows the use of PHP in XML-conformant code
such as XHTML.
The second way is not available always. Short tags are
available only when they have been enabled. This can be
done via the short_tags() function
(PHP 3 only), by enabling the
short_open_tag configuration setting in the PHP config
file, or by compiling PHP with the --enable-short-tags
option to configure. Even if it is
enabled by default in php.ini-dist, use of short tags are
discouraged.
The fourth way is only available if ASP-style tags
have been enabled using the asp_tags
configuration setting.
注: Support for ASP-style tags was added in 3.0.4.
注: Using short tags should be avoided when developing applications or libraries that are meant for redistribution, or deployment on PHP servers which are not under your control, because short tags may not be supported on the target server. For portable, redistributable code, be sure not to use short tags.
The closing tag for the block will include the
immediately trailing newline if one is present. Also, the
closing tag automatically implies a semicolon; you do not
need to have a semicolon terminating the last line of a PHP
block.
PHP allows you to use structures like this: