Which programming language is most widely used for web development?
Web development requires knowledge of markup languages (HTML and XHTML), client-side languages, and server-side languages. The most widely used client-side language is JavaScript - for server-side languages, it is PHP. While these are the most used languages, there are other widely used languages with ample development talent to support them that you could consider (you can see examples of other widely used languages via the links). There are also other tools you may want to consider in making your web development decisions such as whether or not your outsource developers will use a content management system (CMS) such as Wordpress (the most widely used CMS) as part of the development as that will also have implications for future support and development work.
PHP is the most used. However, it is not the best. There are lots of resources for PHP to find to outsource development. Just be prepared for the Pareto principle, 20% of the resources are good 80% are not. Enjoy the adventure