$context = stream_context_create(array(
    'http' => array(
        'method'  => 'POST',
        'content' => $content,
    )
));

$result = file_get_contents('https://webto.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8', null, $context);

$context = stream_context_create([array(
     'http' => array(
         'method'  => 'POST',
         'content' => $content,
     ),
     'ssl' => [
          'crypto_method' => STREAM_CRYPTO_METHOD_TLSv1_1_CLIENT |
                             STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT,
     ],
)]);

$result = file_get_contents('https://webto.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8', false, $context);