How PHP responds to the button's onclick event

How PHP responds to the button's onclick event
I PHP with the CodeIgniter Framework Reference Smart Template, the button event controller on the page how to respond to the expert answer, thank you. Can give an example the best.
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PHP Asynchronous request with JS

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Baidu Ajax synchronous Asynchronous interactive technology
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W3school above the jquery Ajax example is OK
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index.html button and JS execution event

do.php the PHP to be executed, write different content as needed.

 
 $ret = Array ();
$ret [' success '] = 1;
$ret [' msg '] = ' invoke PHP success ';
Header (' Content-type:application/json ');
echo Json_encode ($ret);
?>

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