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How can I calculate values of fields and update the db without leaving or refreshing the PHP page?

sasnaktiv asked
Last Modified: 2020-02-29
How can I calculate values of fields and update the db without leaving or refreshing the PHP page?

Boy do I need help! I've been struggling with this for far too long.
The frustrating thing about it is that I know there's a simple solution because I've done it before -- but I can't find my old code.
So I'm reaching out to guys who know this stuff much better than I do.

I'm merely trying to subtract a value of one field from the value of another field, then inserting the results into a third field, and updating the database -- without leaving or refreshing the page.

Below is a simple example of what it is I'm trying to accomplish.
Any help is greatly appreciated.

<INPUT type=text readonly='readonly' name='[b]AmountBILLED[/b]' value='[b]$AmountBILLED[/b]'> 
[i]NOTE: The value of field '[b]AmountBILLED[/b]' is harvested from a database [/i]

<INPUT type=text name='[b]AmountPAID[/b]' > 
[i]  The value of field '[b]AmountPAID[/b]' is entered by the end user [/i]

[i]  On Focus or Click of a (CALCULATE button) do the math - subtract the value entered in '[b]AmountPAID[/b]' from the value of '[b]AmountBILLED[/b]' and
 insert that value into the '[b]AmountDUE[/b]' field automatically, and UPDATE the db [/i]

<INPUT type=text readonly='readonly' name='[b]AmountDUE[/b]'  value='[b]$AmountDUE[/b]'>
[i] Another option to updating the value of [b]AmountDUE[/b] is to merely do so as a variable that is not in an <INPUT field. [/i]
[i] Thank the Expert who made my life a little easier  [/i]

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PS: I don't know why but the BOLD and ITAL tags which I used for emphasis do not seem to be functioning properly on EE
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