2. XML Syntax

What you will learn

• The rules every XML file must follow
• The XML declaration
• Root elements, nesting, and case
• How to include special characters

The rules

1. There must be one root element that contains everything.
2. Every opening tag needs a closing tag (or be self-closing).
3. Elements must be properly nested.
4. Tag names are case sensitive.
5. Attribute values must be in quotes.
6. Special characters must be escaped.

An XML file that follows these rules is called well-formed. A parser refuses a file that is not.

The XML declaration

An XML file may start with a declaration. It is optional, but good practice. If present, it must be the very first thing in the file, with nothing (not even a blank line) before it.

<?xml version="1.0" encoding="UTF-8"?>

• version is almost always 1.0.
• encoding says how characters are stored. UTF-8 supports every language, and you should save your file as UTF-8 too.

One root element

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book>Learning XML</book>
  <book>Learning PHP</book>
</library>

<?xml version="1.0" encoding="UTF-8"?>
<book>Learning XML</book>
<book>Learning PHP</book>

Closing tags and empty elements

Unlike HTML, every element must be closed. An element with no content may use a self-closing tag.

<note>
  <to>Ali</to>
  <urgent/>
  <attachment></attachment>
</note>

<urgent/> and <attachment></attachment> mean the same thing: an element with no content.

Proper nesting

Close the inner element before the outer one.

<p>This is <b>bold</b> text.</p>

<p>This is <b>bold</p></b>

Case sensitivity

<Message>Hello</message>

Message and message are different names, so these tags do not match, and the file is not well-formed. A common habit is to use lowercase, or camelCase such as firstName, and to stay consistent.

Quotes around attribute values

Attribute values must always be in single or double quotes. (Attributes get their own lesson.)

<note date="2026-03-15">Hello</note>

Special characters

Five characters have a special meaning in XML. To use them in text, write an entity reference instead.

<condition>if salary &lt; 1000 &amp; age &gt; 30</condition>

The characters < and & are strictly forbidden in ordinary text, so always escape them. The others are only required in some places, but escaping them is harmless.

Unlike HTML, XML does not know names such as   or ©. Use the actual character (in a UTF-8 file) or a numeric reference such as ©.

Whitespace

XML keeps whitespace inside text, unlike HTML, which collapses it. Indentation between elements is ignored by most programs, but spaces inside an element’s text can count as data. Indent with two spaces to keep files readable.

Common mistakes

• Two root elements, or text outside the root.
• Forgetting a closing tag, or closing tags in the wrong order.
• Mixing capital letters between opening and closing tags.
• Typing a raw & or < in text.
• Putting a blank line or space before <?xml ... ?>.
• Saving with an encoding that does not match the declaration.

<?xml version="1.0" encoding="UTF-8"?>
<shop>
  <Item>Tom & Jerry</item>
  <price currency=USD>5</price>
</shop>
<note>Thanks</note>

<?xml version="1.0" encoding="UTF-8"?>
<shop>
  <item>Tom &amp; Jerry</item>
  <price currency="USD">5</price>
  <note>Thanks</note>
</shop>

Summary

• A well-formed file has one root, closed and properly nested tags, matching case, and quoted attribute values.
• Start with an optional declaration, and save as UTF-8.
• Escape <, &, and the other special characters with entity references.