$ajax解决方法

$ajax
请教大牛, 如何将js中连接的php执行完后的变量$i传到A.php,让其中被注释掉的那块调用?
还有,在$ajax中success是怎么运行的,回调的是什么值?
谢谢

A.php

		

			

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				Product Line:					
					
				

				

					Design:					
					
				

				

					Technology Engineering:
					
					
				

				

					Simulation:
					
					
				

				

					IMDS
					
					
				

				

					Geometry Testing