What are common issues when using curl in PHP behind a firewall?

When using curl in PHP behind a firewall, a common issue is that the firewall may block outgoing requests made by curl. To solve this, you can set the CURLOPT_PROXY option in curl to route the requests through a proxy server that can bypass the firewall restrictions.

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "http://example.com/api");
curl_setopt($ch, CURLOPT_PROXY, "http://proxy.example.com:8080");
$result = curl_exec($ch);
curl_close($ch);

// Check for errors
if(!$result) {
    echo "Error: " . curl_error($ch);
}