exec - Returns last line of commands output
passthru - Passes commands output directly to the browser
system - Passes commands output directly to the browser and returns last line
shell_exec - Returns commands output
\`\` (backticks) - Same as shell_exec()
popen - Opens read or write pipe to process of a command
proc_open - Similar to popen() but greater degree of control
pcntl_exec - Executes a program