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[yaffs-website] / vendor / nikic / php-parser / doc / 0_Introduction.markdown

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+Introduction
+============
+
+This project is a PHP 5.2 to PHP 7.1 parser **written in PHP itself**.
+
+What is this for?
+-----------------
+
+A parser is useful for [static analysis][0], manipulation of code and basically any other
+application dealing with code programmatically. A parser constructs an [Abstract Syntax Tree][1]
+(AST) of the code and thus allows dealing with it in an abstract and robust way.
+
+There are other ways of processing source code. One that PHP supports natively is using the
+token stream generated by [`token_get_all`][2]. The token stream is much more low level than
+the AST and thus has different applications: It allows to also analyze the exact formatting of
+a file. On the other hand the token stream is much harder to deal with for more complex analysis.
+For example an AST abstracts away the fact that in PHP variables can be written as `$foo`, but also
+as `$$bar`, `${'foobar'}` or even `${!${''}=barfoo()}`. You don't have to worry about recognizing
+all the different syntaxes from a stream of tokens.
+
+Another question is: Why would I want to have a PHP parser *written in PHP*? Well, PHP might not be
+a language especially suited for fast parsing, but processing the AST is much easier in PHP than it
+would be in other, faster languages like C. Furthermore the people most probably wanting to do
+programmatic PHP code analysis are incidentally PHP developers, not C developers.
+
+What can it parse?
+------------------
+
+The parser supports parsing PHP 5.2-5.6 and PHP 7.
+
+As the parser is based on the tokens returned by `token_get_all` (which is only able to lex the PHP
+version it runs on), additionally a wrapper for emulating tokens from newer versions is provided.
+This allows to parse PHP 7.1 source code running on PHP 5.5, for example. This emulation is somewhat
+hacky and not perfect, but it should work well on any sane code.
+
+What output does it produce?
+----------------------------
+
+The parser produces an [Abstract Syntax Tree][1] (AST) also known as a node tree. How this looks like
+can best be seen in an example. The program `<?php echo 'Hi', 'World';` will give you a node tree
+roughly looking like this:
+
+```
+array(
+    0: Stmt_Echo(
+        exprs: array(
+            0: Scalar_String(
+                value: Hi
+            )
+            1: Scalar_String(
+                value: World
+            )
+        )
+    )
+)
+```
+
+This matches the structure of the code: An echo statement, which takes two strings as expressions,
+with the values `Hi` and `World!`.
+
+You can also see that the AST does not contain any whitespace information (but most comments are saved).
+So using it for formatting analysis is not possible.
+
+What else can it do?
+--------------------
+
+Apart from the parser itself this package also bundles support for some other, related features:
+
+ * Support for pretty printing, which is the act of converting an AST into PHP code. Please note
+   that "pretty printing" does not imply that the output is especially pretty. It's just how it's
+   called ;)
+ * Support for serializing and unserializing the node tree to XML
+ * Support for dumping the node tree in a human readable form (see the section above for an
+   example of how the output looks like)
+ * Infrastructure for traversing and changing the AST (node traverser and node visitors)
+ * A node visitor for resolving namespaced names
+
+ [0]: http://en.wikipedia.org/wiki/Static_program_analysis
+ [1]: http://en.wikipedia.org/wiki/Abstract_syntax_tree
+ [2]: http://php.net/token_get_all