• source: ​https://lists.ms4w.com/pipermail/ms4w-users/2019-May/000343.html
• note: likely solution is included in next MapServer release:

SWIG MapScript (currently fails)

include("../includes/phpmapscriptng-swig/mapscript.php");
$oMap= new mapObj($mymapfile);

msIO_installStdoutToBuffer();

$request = new OWSRequest();

//Get Request
foreach ($_GET as $k=>$v) {
    $request->setParameter($k, $v);
}

$oMap->OWSDispatch($request);

unset($oMap);

$contenttype = strtolower(msIO_stripStdoutBufferContentType());

//Capabilities Request
if ($contenttype == "application/vnd.ogc.wms_xml; charset=utf-8"){
    header('Content-type: application/xml; charset=utf-8');
    echo msIO_getStdoutBufferString();
}

//getMap jpeg
else if ($contenttype == 'image/jpeg'){
    header('Content-type: image/jpeg');
    echo msIO_getStdoutBufferBytes();
}

Old MapScript

$oMap= ms_newMapObj($mymapfile);

ms_ioinstallstdouttobuffer();

$request = ms_newowsrequestobj();

//Get Request
foreach ($_GET as $k=>$v) {
    $request->setParameter($k, $v);
}

$oMap->owsdispatch($request);
$oMap->free();
unset($oMap);

$contenttype = strtolower(ms_iostripstdoutbuffercontenttype());

//Capabilities Request
if ($contenttype == "application/vnd.ogc.wms_xml; charset=utf-8"){
    header('Content-type: application/xml; charset=utf-8');
    echo ms_iogetstdoutbufferstring();
}

//getMap jpeg
else if ($contenttype == 'image/jpeg'){
    header('Content-type: image/jpeg');
    echo ms_iogetStdoutBufferBytes();
}