What is not mentioned in the documentation is that if you want to set  a local-port or local-port-range to establish a connection is possible by adding   CURLOPT_LOCALPORT and CURLOPT_LOCALPORTRANGE options.   

Ex:
$conn=curl_init ('example.com');
curl_setopt($conn, CURLOPT_LOCALPORT, 35000);
curl_setopt($conn, CURLOPT_LOCALPORTRANGE, 200);

CURLOPT_LOCALPORT: This sets the local port number of the socket used for the connection.

CURLOPT_LOCALPORTRANGE:  The range argument is the number of attempts libcurl will make to find a working local port number. It starts with the given CURLOPT_LOCALPORT and adds one to the number for each retry. Setting this option to 1 or below will make libcurl do only one try for the exact port number.

Interface can be also configured using CURLOPT_INTERFACE:

Ex:

curl_setopt($conn,  CURLOPT_INTERFACE, "eth1");