Quick chart that helped me immensely (Best (+) / worst (o) of each command) and when to use each (w).
| exec(string cmd, [array cmd_output, [int cmd_exit_status]])
+- Second (optional) argument holds an array containing the output line by line.
+- Third (optional) argument holds the exit status of the executed program.
w- When the output of a command is only important to the program.
| system(string cmd, [int cmd_exit_status])
+- Echos ascii output straight to browser (only stdout; stderr must be redirected by putting 2>&1 after the command to see errors).
+- Second (optional) argument holds the exit status of the executed program.
w- When the output of a command is important to the user.
The more I go over my notes the less I care for the last two functions.
Shell_exec() = exec() + implode() - exit codes.
And while neccessary to be able to pipe binary through php, most people will never use passthru
| passthru(string cmd, [int cmd_exit_status])
+- Echos binary output directly to browser window.
+- Second (optional) argument holds the exit status of the executed program.
w- When the output of a command is binary.
| shell_exec(string cmd) <-- minimal features to maintain sync with normal use of backticks.
+- Returns full output as a string instead of directly to the browser (only stdout; stderr must be redirected).
o- No way to check exit status (Should use system() with both stderr and stdout redirected to /dev/null to get the exit code).