Migrating from PHP 5.1.x to PHP 5.2.x

• What has changed in PHP 5.2.x
• Backward Incompatible Changes
• New Error Messages
• Changes in PHP datetime support
• New Parameters
• New Functions
• New Methods
• Removed Extensions
• New Extensions
• New Classes
• New Global Constants
• New Class Constants
• New INI Configuration Directives
• Error Reporting
• Other Enhancements

See also the migration guides for PHP versions 5.0.x, 5.1.x, 5.3.x, 5.4.x, 5.5.x and 5.6.x.