Rename file extensions with Grunt copy.

Gruntfile.js

// Compile Jade (allowing for php)
jadephp: {
  options: {
    pretty: true
  },
  // Compile templates to HTML in .tmp so we can run useminPrepare
  dist: {
    files: [
      {
        expand: true,
        cwd: '<%= config.app %>/templates',
        dest: '.tmp/templates',
        src: '*.jade',
        ext: '.html'
      },
    ]
  },
},

copy: {
  dist: {
    files: [
      {
        // Copy HTML tempaltes and convert them to php
        expand: true,
        dot: true,
        cwd: '.tmp/templates',
        dest: '<%= config.dist %>/templates',
        src: '*.html',
        ext: '.php' // Change the file ext
      }
    ]
  }
},

grunt.registerTask('build', [
  'jadephp:dist',
  'useminPrepare',
  'concat',
  'uglify',
  'cssmin',
  'usemin' // Performs the rewrites
  'copy:dist' // Moves the files and changes the ext to .php
]);