According to the ChangeLog, the inablity to fork issue in the exec family has been fixed on Win32 in 4.0.6.  Unfortunately, this is not so.  Calling system(), exec(), or passthru() produces the following error msg:

Warning: Unable to fork [test.mp3] in exec.php at line 3

Calling escapeshellcmd() or escapeshellarg() does not give an error or warning message, but also does not execute the command.  The above mentions problems occur when the functions are called when running PHP as an Apache module, or by executing the script from command line.  Below is a description of my setup, please let me know if additional information is needed.

OS: Win98 - SE
Processor: AMD K6-2/350
RAM: 192 megs
Apache version: 1.3.20 (run as service)
PHP: 4.0.6 and 4.0.5 (with Zend Optimizer)
Enabled PHP extensions: php_bz2.dll, php_gd.dll, php_imap.dll, php_zlib.dll
Output Buffering: On
PHP-GTK: version 0.0.4