Thanks for the suggestions I got it to work for me in the end.
$var1= explode(",", variablestring,variablestring,variablestring);
$myglue = extrastringinfo;
$var2= implode ("$myglue", $var1);
$echo $var2
Outputs:
variablestringextrastringinfo,variablestringextrastringinfo,variablestringextrastringinfo
Funny how the simple things seem to trip you up..🙂