This uses pandoc to do the heavy lifting. Also note that if all you want is a markdown view and you're happy with e.g google-chrome, then consider the following

#!/bin/bash
A="$(mktemp --suffix=.html)"
pandoc --quiet -f markdown_phpextra -t html -s "$@" >| "$A" && google-chrome "$A"

and also for arbitrary input formats:

#!/bin/bash
L="$1"
shift
A="$(mktemp --suffix=.html)"
pandoc --quiet -f "$L" -t html -s "$@" >| "$A" && google-chrome "$A"

It would be an easy exercise to modify the Python below to allow arbitrary languages in a similar way.

The source

#!/usr/bin/env python
from PySide6.QtCore import *
from PySide6.QtGui import *
from PySide6.QtWidgets import *
from PySide6.QtWebEngineWidgets import *
from subprocess import run, PIPE
import sys

args = sys.argv[1:]

if len(args) == 0:
  args = ["-"]
# cache stdin if - is one of the args
# doing it this way causes multiple -'s to duplicate stdin
# rather than reading from stdin multiple times
if "-" in args:
  stdin = sys.stdin.read()
markdown = ""
for x in args:
  if x == "-":
    markdown += stdin
  else:
    try:
      with open(x) as f:
        markdown += f.read()
    except Exception:
      print(f"Failed to read {x}",file=sys.stderr)
      continue
  markdown += "\n\n"

m = run(["pandoc","--quiet","-f","markdown","-t","html","-s"],input=markdown.encode(),stdout=PIPE,stderr=PIPE)
if m.returncode != 0:
  print(f"#Fail markdown")
  exit(1)
html = m.stdout.decode()

class View(QWidget):
  def __init__(self,html):
    super().__init__()
    self.webview = QWebEngineView(self)
    self.webview.setHtml(html)
    self.webview.resize(self.rect().size())
  def resizeEvent(self,e):
    self.webview.resize(self.rect().size())
  def keyPressEvent(self,e):
    if e.text().lower() == "q":
      app.quit()

app = QApplication()
view = View(html)
view.show()

app.exec()

To allow for arbitrary languages, I do (where source is now in the variable source instead of markdown)

args = sys.argv[1:]
lang = "markdown_phpextra"
if len(args) == 0:
  args = ["-"]
if len(args) > 1:
  lang = args.pop(0)
...
m = run(["pandoc","--quiet","-f",lang,"-t","html","-s"],input=source.encode(),stdout=PIPE,stderr=PIPE)