PHP 3 includes support for a network-based debugger. PHP 4 does not have an internal debugging facility. You can use one of the external debuggers though. The Zend IDE includes a debugger, and there are also some free debugger extensions like DBG at &url.dbg;, the Advanced PHP Debugger (APD) or Xdebug which even has a compatible debugger interface as PHP 3's debugging functionality as is described in this section. The internal debugger in PHP 3 is useful for tracking down evasive bugs. The debugger works by connecting to a TCP port for every time PHP 3 starts up. All error messages from that request will be sent to this TCP connection. This information is intended for "debugging server" that can run inside an IDE or programmable editor (such as Emacs). How to set up the debugger: Set up a TCP port for the debugger in the configuration file (debugger.port) and enable it (debugger.enabled). Set up a TCP listener on that port somewhere (for example socket -l -s 1400 on Unix systems). In your code, run "debugger_on(host)", where host is the IP number or name of the host running the TCP listener. Now, all warnings, notices etc. will show up on that listener socket, even if you turned them off with error_reporting. The PHP 3 debugger protocol is line-based. Each line has a type, and several lines compose a message. Each message starts with a line of the type start and terminates with a line of the type end. PHP 3 may send lines for different messages simultaneously. A line has this format: date Date in ISO 8601 format (yyyy-mm-dd) time Time including microseconds: hh:mm:uuuuuu host DNS name or IP address of the host where the script error was generated. pid PID (process id) on host of the process with the PHP 3 script that generated this error. type Type of line. Tells the receiving program about what it should treat the following data as: Name Meaning start Tells the receiving program that a debugger message starts here. The contents of data will be the type of error message, listed below. message The PHP 3 error message. location File name and line number where the error occurred. The first location line will always contain the top-level location. data will contain file:line. There will always be a location line after message and after every function. frames Number of frames in the following stack dump. If there are four frames, expect information about four levels of called functions. If no "frames" line is given, the depth should be assumed to be 0 (the error occurred at top-level). function Name of function where the error occurred. Will be repeated once for every level in the function call stack. end Tells the receiving program that a debugger message ends here. data Line data. Debugger PHP 3 Internal warning E_WARNING error E_ERROR parse E_PARSE notice E_NOTICE core-error E_CORE_ERROR core-warning E_CORE_WARNING unknown (any other)