Here's an updated version of the PHP script, and a new example Typoscript, that tells the PHP script to look for a GET var, and use that value to determine which "page" should be set to active.

In the Typoscript for the userFunc, you can now set 2 extra parameters:

obj.header.special.userFunc.actextkey = tx_ablinklist_pi1

Which sets the extension key of the GET var that should be used to determine the ACT state.

And:

obj.header.special.userFunc.actextvar = category_uid

Which sets the extension variable of the GET var that should be used to determine the ACT state.

The new Typoscript Example:

-- cut -- - ERROR: Line 1: Object Name String, "--" was not preceded by any operator, =<>({;Line 43: Object Name String, "--" was not preceded by any operator, =<>({
page.includeLibs.menustuff = fileadmin/menuFunc2.inc

obj.header = HMENU
obj.header.special = userfunction
obj.header.special.userFunc = user_menuFunc->makeMenuArray

# Set the "table" to the DB table you want to use for the menu.
# In this example I use the Modern Linklist (ab_linklist) extension.
obj.header.special.userFunc.table = tx_ablinklist_category

# Here you set the "pidInList" to the location of your DB records.
obj.header.special.userFunc.select.pidInList = 91

# Set "actextkey" to the extension key of the GET var that
# should be used to determine the ACT state.
obj.header.special.userFunc.actextkey = tx_ablinklist_pi1

# Set "" to the extension variable of the GET var that
# should be used to determine the ACT state.
obj.header.special.userFunc.actextvar = category_uid
obj.header.1 = TMENU
obj.header.1 {
  NO {
    doNotLinkIt = 1
    stdWrap.field = label
    stdWrap.typolink.useCacheHash = 1
    stdWrap.typolink.parameter.data = page:uid
    stdWrap.typolink.additionalParams.dataWrap = &tx_ablinklist_pi1[category_uid]={field:uid}
    #stdWrap.data = debug:data
    allWrap = |<br>
  }
  ACT < .NO - ERROR: Line 75: Object copied in this line "ACT < .NO" would leave either the value or properties untouched in TypoScript Version 1. Please check that this is not a problem for you.
  ACT = 1
  ACT.allWrap = <b>|</b><br>
}

page.7911 < obj.header
-- cut -- - ERROR: Line 39: Object Name String, "--" was not preceded by any operator, =<>({;Line 81: Object Name String, "--" was not preceded by any operator, =<>({