how to call php function in read data - Fields to retrieve box

I need to generate a new field value from fields read from a database. using a PHP function
Something like

badgetext= getBadgeDisplay($red, $green, $blue, $opacity) 

where the parameters are read from the database record.
So I assume that I should add the fields to retrieve box as an extra field like

mdlMasterYears.badgeText:getBadgeDisplay(red, green, blue, opacity)

where red, green, blue, opacity are read fro m the database record.
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That does not work. How should I do that short of looping through the records after the read in a separate function? What is that Special Fields box for? Is that a possibility?
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Regards
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Nick

Why not read the row you want from the database, and then process it in a PHP function to the value you want? Not going to be that computationally burdensome to just loop through the results.