Have you tried passing a whole point-string for each placeholder?

So something like:

$arg =' POINT ( -4.67682813534559 58.3335955352018 ) '
-- then using execute($arg) for a statement with 
ST_GeomFromText( ? )

--
-- (maybe casting the argument to WKT, if such casting is supported)
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(just curious, it probably won't work either...)