test-plugin.js

const fs = require('fs')

module.exports = function (babel) {
  return {
    visitor: {
      ExportNamedDeclaration(path, state) {
        const filename = state.file.opts.filename;
        const name = path.node.declaration.declarations[0].id.name
        if (name !== 'php') return

        const code = path.node.declaration.declarations[0].init.quasis[0].value.raw
        const phpFilePath = filename.replace(/^.*[\\\/]/, '').replace('.js', '.php')
        fs.writeFileSync(`./build/${phpFilePath}`, code)
      },
    }
  }
}