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Null

The null element represents a NULL value or empty string. This is the element's syntax:

<null/>

boolean

The boolean element represents a Boolean value. The value of this element: true or false. This is case sensitive, so, use lowercase.

<boolean>true</boolean>

number

The number element contain the serialized value for the number 12345, -12345, 12.345, -12.345, and 123456789012345:

<number>12345</number>
<number>-12345</number>
<number>12.345</number>
<number>-12.345</number>
<number>123456789012345</number>

dateTime

The dateTime element carries date and time information in ISO 8601 format. Although PHP doesn't have a native datetime type, we can use date() function with the c format parameter: date('c'). Another alternative is add DATE_ISO8601 constant in PHP 5.1

<dateTime>2008-06-25T15:20:04-04:00</dateTime>

string

The string element contains arbitrary length string that must not contain embedded NULLs.

<string>It is easy to learn php with phpeveryday</string>