Introduction and use of xml files

We often see xml files, but rarely use them. As the name suggests, xml is an extensible markup language. Its development is shown in the figure below:

Comparison of xml and html:

Comparison Content	HTML	XML
Extensibility	Does not allow users to define their own extension tags	Allows users to define their own extension tags
Structure description	Does not support deep nested expressions	Supports deep nested expressions, suitable for representing document data with complex structures
Readability and maintainability	Difficult to read and maintain	Clear structure, easy to read and maintain
Data and display Relationship	Content description and presentation form are integrated	Content description and presentation form are separated

xml is more powerful than html, but its use is more standardized and strict:

(1). XML is case-sensitive;

(2). The starting and ending tags of all elements must appear in pairs and be correct Nesting;

(3). If an XML description is used, it must be the first line of the XML document:

(4). Element attributes must be in quotes Quotes can be single or double quotes, but they must appear in pairs. Such as:

(5). XML naming rules:

①.XML names start with an underscore or letter;

②.XML names can contain letters, numbers, periods, underscores and colons;

③.XML names cannot contain spaces;

④.XML names cannot start with numbers, but can contain numbers;

⑤.XML names are size-sensitive Write.

(6)． Retain markup characters. If you want to display tags like

<: represents the character>

> ;: Represents >character

&: Represents & character

&apos: Represents 'character

": Represents "character

We can also use ENTITY custom entities:

In this way we can call it with &linux;.

(7). The spaces in the XML document content are meaningful and will be retained after conversion.

(8). Elements start with , such as

Look at a simple xml code:

Understand the concepts of elements, attributes and entities:

Element It is a tag such as the classes element; the attribute is additional information such as the englishname attribute of the name element; the entity is used to replace the string in xml, such as When using www.phpddt.com You can use the &website cross-border method!

DTD's "Document Type Definition" introduction:

standardizes the format of XML files, such as:

mckee phpddt1990@163.com
DTD is actually one or several files written in XML.

Each item in the document type declaration is an element declaration, defining the constraints of each element. The format of the element declaration is:

The content includes:

1. #PCDATA, which specifies that the element only contains parsed character data.

2. Child elements can specify the child elements of the element. You can also use commas as delimiters to specify multiple sub-elements. And the order in which subelements appear must be in the order in which they are defined.

3. The number of sub-elements, we can specify the number of sub-elements through regular expressions. ①?, allows zero or one of this element ②*, allows zero or more of this element ③+, allows one or more of this element

4. Optional (|), option is a parameter list, each parameter Separated by "|", it means that only one child element can be selected.

5. Parentheses, you can use parentheses to enclose the options to express a richer meaning. For example, we want to indicate that the choice element must contain a good sub-element, and must contain either an ok sub-element or a bad sub-element.

6. Mixed content. In some documents, an element may contain both sub-elements and strings. These contents are called mixed content.

7. Empty elements. Some elements do not need to contain any content and are called empty elements. Written as a separate tag ending with />.

8. ANY, allows elements to contain any content. This option is useful during dtd testing, but should be avoided in production systems.

A valid XML document must declare the attributes of the element. This is done using the ATTLIST declaration. An ATTLIST can declare multiple attributes for an element type.

Every element used in a valid document must be declared with an element declaration in the document's DTD. element_name can be any legal XML name, and content_model (content model) specifies the child elements that the element can or must contain and the order of the child elements.

Important note:

1.xml can display styles, and you need to import the css file

2. The above xml file , xml has only one root element, which is the first element!

3. Used to display special characters

4. Annotation method

The above is the introduction and use of xml files , for more related content, please pay attention to the PHP Chinese website (www.php.cn)!