You need to use "Execute()" to call functions using variable names.

Here is an example:
Code:
Break ON
$=SetOption("Explicit","ON")
 
Dim $s
 
$s="This is a string"
 
"Start string is: '"+$s+"'"+@CRLF
"      In hex is: '"+udfFunction("udfHexString($$sParam1)",$s)+"'"+@CRLF
"    Reversed is: '"+udfFunction("udfReverse($$sParam1)",$s)+"'"+@CRLF
 
Function udfFunction($sFunctionToCall,$sParam1)
	Dim $sDiscard
	$sDiscard=Execute("$$udfFunction="+$sFunctionToCall)
	Exit @ERROR
EndFunction
 
Function udfHexString($s)
	While $s<>""
		$udfHexString=$udfHexString+Right("00"+DecToHex(Asc($s)),2)
		$s=SubStr($s,2)
	Loop
EndFunction
 
Function udfReverse($s)
	While $s<>""
		$udfReverse=Left($s,1)+$udfReverse
		$s=SubStr($s,2)
	Loop
EndFunction