Suspends execution of the current process until a child as specified by the process_id argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function.
If a child as requested by process_id has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed. Please see your system's waitpid(2) man page for specific details as to how waitpid works on your system.
The value of process_id can be one of the following:
possible values for process_id
< -1 wait for any child process whose process group ID is equal to the absolute value of process_id. -1 wait for any child process; this is the same behaviour that the wait function exhibits. 0 wait for any child process whose process group ID is equal to that of the calling process. > 0 wait for the child whose process ID is equal to the value of process_id.
Specifying -1 as the process_id is equivalent to the functionality pcntl_wait provides (minus flags).
pcntl_waitpid will store status information in the status parameter which can be evaluated using the following functions: pcntl_wifexited, pcntl_wifstopped, pcntl_wifsignaled, pcntl_wexitstatus, pcntl_wtermsig and pcntl_wstopsig.
The value of flags is the value of zero or more of the following two global constants OR'ed together:
possible values for flags
WNOHANG return immediately if no child has exited. WUNTRACED return for children which are stopped, and whose status has not been reported.
pcntl_waitpid returns the process ID of the child which exited, -1 on error or zero if WNOHANG was used and no child was available