The package definition file package.xml is, as the name already implies, a well-formed XML file that contains all information about a PEAR package. Version 2.0 contains several important enhancements over version 1.0, including

Channel support
Binary PECL packages support (not fully implemented in PEAR 1.4.0)
More specific dependency resolution

For those of you with an existing package.xml version 1.0, you can create an approximate equivalent package using the

$ pear convert

command. Note that as of version 1.6.0, PEAR_PackageFileManager supports package.xml 2.0 with the PEAR_PackageFileManager2 class.

PECL developers: for more information on pecl-specific features, read here.

Note: package.xml version 2.0 is supported from PEAR 1.4 on. As of 2007-04, PEAR 1.4 or greater is used in more than 99.8% of all installations in the wild that downloaded packages - there is no need to support v1.0 for backwards compatibility anymore.

An example file with all elements

<?xml version="1.0"?>
<package version="2.0" xmlns="http://pear.php.net/dtd/package-2.0"
    xmlns:tasks="http://pear.php.net/dtd/tasks-1.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://pear.php.net/dtd/tasks-1.0
http://pear.php.net/dtd/tasks-1.0.xsd
http://pear.php.net/dtd/package-2.0
http://pear.php.net/dtd/package-2.0.xsd">
 <name>PEAR</name>
 <channel>pear.php.net</channel>
 <summary>Any one-line summary</summary>
 <description>any static long description.
 This text should not change very much between releases, use the "notes" tag
 for release notes
 </description>
 <lead>
  <name>Greg Beaver</name>
  <user>cellog</user>
  <email>cellog@php.net</email>
  <active>yes</active>
 </lead>
 <date>2005-02-26</date>
 <time>20:30:13</time> <-- note: <time> is optional -->
 <version>
  <release>1.4.0a2</release>
  <api>1.4.0</api>
 </version>
 <stability>
  <release>alpha</release>
  <api>alpha</api>
 </stability>
 <license uri="http://www.php.net/license">PHP License</license>
 <notes>
 Put release notes here.
 They can be multi-line
 </notes>
 <contents>
  <dir name="/">
   <dir name="PEAR">
    <dir name="ChannelFile">
     <file name="Parser.php" role="php" />
    </dir> <!-- /PEAR/ChannelFile -->
    <file name="Dependency2.php" role="php">
     <tasks:replace from="@PEAR-VER@" to="version" type="package-info"/>
    </file>
   </dir> <!-- /PEAR -->
   <dir name="scripts" baseinstalldir="/">
    <file name="pear.bat" role="script">
     <tasks:replace from="@bin_dir@" to="bin_dir" type="pear-config" />
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
     <tasks:windowseol/>
    </file>
    <file name="pecl.bat" role="script">
     <tasks:replace from="@bin_dir@" to="bin_dir" type="pear-config" />
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
     <tasks:windowseol/>
    </file>
    <file name="pear.sh" role="script">
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@php_dir@" to="php_dir" type="pear-config" />
     <tasks:replace from="@pear_version@" to="version" type="package-info" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
     <tasks:unixeol/>
    </file>
    <file name="pecl.sh" role="script">
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@php_dir@" to="php_dir" type="pear-config" />
     <tasks:replace from="@pear_version@" to="version" type="package-info" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
     <tasks:unixeol/>
    </file>
    <file name="pearcmd.php" role="php">
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@php_dir@" to="php_dir" type="pear-config" />
     <tasks:replace from="@pear_version@" to="version" type="package-info" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
    </file>
    <file name="peclcmd.php" role="php">
     <tasks:replace from="@php_bin@" to="php_bin" type="pear-config" />
     <tasks:replace from="@php_dir@" to="php_dir" type="pear-config" />
     <tasks:replace from="@pear_version@" to="version" type="package-info" />
     <tasks:replace from="@include_path@" to="php_dir" type="pear-config" />
     <tasks:footask/>
    </file>
   </dir> <!-- /scripts -->
   <file name="package.dtd" role="data" />
   <file name="postinstall.php" role="php">
    <tasks:postinstallscript/>
   </file>
   <file name="template.spec" role="foo" />
  </dir> <!-- / -->
 </contents>
 <compatible>
  <name>FooPackage</name>
  <channel>pear.php.net</channel>
  <min>1.3.0</min>
  <max>1.5.0</max>
 </compatible>
 <dependencies>
  <required>
   <php>
    <min>4.2</min>
    <max>6.0.0</max>
   </php>
   <pearinstaller>
    <min>1.4.0dev13</min>
   </pearinstaller>
   <package>
    <name>Archive_Tar</name>
    <channel>pear.php.net</channel>
    <min>1.1</min>
    <recommended>1.2</recommended>
   </package>
   <package>
    <name>Foo</name>
    <uri>http://www.example.com/Foo-1.2.0.tgz</uri>
   </package>
   <extension>
    <name>xml</name>
   </extension>
   <os>
    <name>windows</name>
    <conflicts/>
   </os>
   <arch>
    <pattern>*-i?86-*-*</pattern>
   </arch>
  </required>
  <optional>
   <package>
    <name>PEAR_Frontend_Web</name>
    <channel>pear.php.net</channel>
    <min>0.5.0</min>
   </package>
   <package>
    <name>PEAR_Frontend_Gtk</name>
    <channel>pear.php.net</channel>
    <min>0.4.0</min>
   </package>
  </optional>
  <group name="remoteinstall" hint="adds the ability to install packages to a remote ftp server">
   <package>
    <name>Net_FTP</name>
    <channel>pear.php.net</channel>
    <min>1.3.0RC1</min>
    <recommended>1.3.0</recommended>
   </package>
  </group>
  <group name="webinstaller" hint="PEAR's web-based installer">
   <package>
    <name>PEAR_Frontend_Web</name>
    <channel>pear.php.net</channel>
    <min>0.5.0</min>
   </package>
  </group>
  <group name="gtkinstaller" hint="PEAR's PHP-GTK-based installer">
   <package>
    <name>PEAR_Frontend_Gtk</name>
    <channel>pear.php.net</channel>
    <min>0.4.0</min>
   </package>
  </group>
 </dependencies>
 <usesrole>
  <role>foo</role>
  <package>Foo</package>
  <channel>pear.example.com</channel>
 </usesrole>
 <usestask>
  <task>footask</task>
  <package>Footask</package>
  <channel>pear.example.com</channel>
 </usestask>
 <phprelease>
  <installconditions>
   <os>
    <name>windows</name>
   </os>
  </installconditions>
  <filelist>
   <install as="pear.bat" name="scripts/pear.bat" />
   <install as="pecl.bat" name="scripts/pecl.bat" />
   <install as="pearcmd.php" name="scripts/pearcmd.php" />
   <install as="peclcmd.php" name="scripts/peclcmd.php" />
   <ignore name="scripts/pear.sh" />
   <ignore name="scripts/pecl.sh" />
  </filelist>
 </phprelease>
 <phprelease>
  <filelist>
   <install as="pear" name="scripts/pear.sh" />
   <install as="pecl" name="scripts/pecl.sh" />
   <install as="pearcmd.php" name="scripts/pearcmd.php" />
   <install as="peclcmd.php" name="scripts/peclcmd.php" />
   <ignore name="scripts/pear.bat" />
   <ignore name="scripts/pecl.bat" />
  </filelist>
 </phprelease>
 <changelog>
  <release>
   <version>
    <release>1.3.5</release>
    <api>1.3.0</api>
   </version>
   <stability>
    <release>stable</release>
    <api>stable</api>
   </stability>
   <date>2005-02-26</date>
   <license uri="http://www.php.net/license/3_0.txt">PHP License</license>
   <notes>
 * fix Bug #3505: pecl can't install PDO
 * enhance pear run-tests dramatically
 * fix Bug #3506: pear install should export the pear version into the environment

   </notes>
  </release>
  <release>
   <version>
    <release>1.4.0a1</release>
    <api>1.4.0</api>
   </version>
   <stability>
    <release>alpha</release>
    <api>alpha</api>
   </stability>
   <date>2005-02-26</date>
   <license uri="http://www.php.net/license/3_0.txt">PHP License</license>
   <notes>
  This is a major milestone release for PEAR.  In addition to several killer features,
  every single element of PEAR has a regression test, and so stability is much higher
  than any previous PEAR release, even with the alpha label.

  New features in a nutshell:
  * full support for channels
  * pre-download dependency validation
  * new package.xml 2.0 format allows tremendous flexibility while maintaining BC
  * support for optional dependency groups and limited support for sub-packaging
  * robust dependency support
  * full dependency validation on uninstall
  * support for binary PECL packages
  * remote install for hosts with only ftp access - no more problems with
    restricted host installation
  * full support for mirroring
  * support for bundling several packages into a single tarball
  * support for static dependencies on a url-based package

  Specific changes from 1.3.5:
  * Implement request #1789: SSL support for xml-rpc and download
  * Everything above here that you just read
   </notes>
  </release>
 </changelog>
</package>