What are the differences of system(), exec() and shell_exec() in PHP?

It is possible to run an external command by three PHP functions of

system();
exec();
shell_exec();

but what are their differences? In spite of their specific applications, in most cases, the can be equally used. I am curious to know which is preferred one when they can be equally used. For example, for unzipping a file or compressing a folder (with tar command), which one is preferred (probably from performance point of view)?

UPDATE: In another question, I found a very useful link describing different aspects for these functions. I share the link here, as other may use to better understand security issues and other aspects.

exec — Execute an external program

system — Execute an external program and display the output

shell_exec — Execute command via shell and return the complete output as a string

so if you don't need the output, I would go with exec.

Further details:

• http://php.net/manual/en/function.exec.php
• http://php.net/manual/en/function.system.php
• http://php.net/shell_exec