Update ph7_cgi.c

Add examples/ph7_cgi.c.

Fix handle leak.

Update README.md

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Introduction to the implementation of foreign functions plus the related interfaces such as ph7_value_to_int(),  ph7_result_string() and so on.

Introduction to the powerful constant expansion mechanism introduced by the version 2 of the PH7 engine.

The PH7 interpreter is a simple,basic stand-alone PHP interpreter that allows
the user to enter and execute PHP files against a PH7 engine. 
To start the ph7 program, just type "ph7" followed by the name of the PHP file
to compile and execute. That is, the first argument is to the interpreter, the rest
are scripts arguments, press "Enter" and the PHP code will be executed.
If something goes wrong while processing the PHP script due to a compile-time
error,your error output (STDOUT) should display the compile-time error messages.