What is the API to get Bitcoin's real-time Buy price and sell price on my site? and how can get it from to implement in my website developed in PHP.
Bitcoin does not have one single buy and sell price. Bitcoins are exchanged on many different exchanges, each keeping track of their own market price.
However, BitcoinCharts.com has a useful API that summarises many of the largest exchanges according to their trading volume. You could use that to display the Bitcoin price on your website.
Other interesting Bitcoin Price APIs that you might consider are those currently provided by Quandl and BitcoinAverage.
Check out 1Forge, they provide a free JSON API for several cryptocurrencies (Bitcoin, Etherium, LiteCoin, Ripple, Dash, and Bitcoin Cash). They also provide data for several standard Forex pairs + gold and silver.
Have a look at their documentation here: https://1forge.com/forex-data-api/api-documentation