Filter one combobox value from another
i am currently working in php but unlike asp.net (postback), in php i want to filter one combobox value from another combobox. Let's say, the at the top i have vendors (combobox) and below i have categories combobox. Now, as user select the top one (selecte particular vendor), it should populate the appropriate categories from the database (mysql) and show them in category combobox.

And please! show proper example. I will really appreciate someone's work.  
Thanks in advance.
Filter one combobox value from another
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i am currently working in php but unlike asp.net (postback), in php i want to filter one combobox value from another combobox. Let's say, the at the top i have vendors (combobox) and below i have categories combobox. Now, as user select the top one (selecte particular vendor), it should populate the appropriate categories from the database (mysql) and show them in category combobox.

And please! show proper example. I will really appreciate someone's work.  
Thanks in advance.