PHP is a sufficiently rich programming environment that it is not common that I truly need to execute external programs on the server on which it executes. However, every once in a while, this situation does come along, and for these, it is important to understand the options that PHP provides, what their differences are, and their relative strengths and weaknesses.

There are four primary choices for executing external programs in PHP:

• The system function.
• The exec function.
• The shell_exec function or its syntactic analogue, the backtick operator, ( ` ).
• the passthru function.