Why is the variable's value not substituted in the string?

[tigueraje]

Hai, can anyone help me...When I write like: \[code\]$file=file_get_contents('https://graph.facebook.com/me?access_token=1932993|twetrg|vsfgsewr');\[/code\]the code gets response as well good.but when I write like:\[code\]1. $tk='';2. $tk='1932993|twetrg|vsfgsewr';//intialize the token value to variable3. $file=file_get_contents('https://graph.facebook.com/me?access_token=$tk');\[/code\]then the Line 3. display warning as "failed to open stream: HTTP request failed! "kindly help me