PHP is a programming language for creating websites and web applications, therefore, you can earn with PHP by creating websites and web applications. Make a website for yourself, make several websites for your friends for free or just for pennies to compose initial portfolio and post it on your web site. Then start actively look for paid projects on freelance marketplaces, forums, etc. But beware that the PHP-development market is very saturated and you will have many experienced competitors with amazing portfolios.
you can create web based solution by using PHP as its offer most security. best example is facebook, facebook has designed by PHP language. You can look for freelancing sites to get PHP projects