Description:
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Using popen to execute a program such as /usr/lib/sendmail when running PHP in safe mode and with the safe_mode_exec_dir directive being null, PHP still attempts to execute the named program in the root directory.  I believe this has the potential to be exploited as a hacking mechanism if the behavior is not changed.

Reproduce code:
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use popen in safe mode to try and execute /usr/lib/sendmail.  PHP will return a 127 error.  If, however, you put a symbolic link named sendmail in the root directory that points to /usr/lib/sendmail, PHP will execute the program perfectly.

Expected result:
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If the safe_mode_exec_dir directive in the PHP.ini file has a null value, then it would seem proper to not allow PHP to execute any program via a popen.  Only if there is a value associated with the safe_mode_exec_dir directive should PHP actually execute a program.

As an additional suggestion, the safe_mode_exec_dir directive could be defaulted to some directory such as "/usr/php_safe_exec" where users could place links to programs outside the safe exec directory.  This would seem to be a much more secure solution than encouraging a user to place /usr/lib, /usr/bin, /use/sbin, or other directory that may contain executables that could be used to compromise the system if an entire pre-populated system directory was placed into this directive.