paddle: Add a test for `php_serialize()`

The expected value was generated with the following PHP code, some of
which was reproduced from the PHP example on
https://paddle.com/docs/reference-verifying-webhooks/ :

    <?php
    	$fields = array(
    		'checkout_id' => '1234asdfjkl',
    		'currency' => 'USD',
    		'customer_name' => 'Senjougahara',
    	);

    	ksort($fields);
    	foreach($fields as $k => $v) {
    		if(!in_array(gettype($v), array('object', 'array'))) {
    			$fields[$k] = "$v";
    		}
    	}

    	$data = serialize($fields);
    	echo "Data: ", $data;

1 files changed, 12 insertions, 0 deletions

diff --git a/license-generator/paddle/src/lib.rs b/license-generator/paddle/src/lib.rs
index a1db417..ebb0f01 100644
--- a/license-generator/paddle/src/lib.rs
+++ b/license-generator/paddle/src/lib.rs
@@ -32,9 +32,21 @@ where I: ExactSizeIterator<Item = (&'a str, &'a str)> {
 
 #[cfg(test)]
 mod tests {
+    use std::collections::BTreeMap;
+
     use super::*;
 
     #[test]
     fn php_serialize_serializes_key_values_as_php_string() {
+        let mut data = BTreeMap::new();
+        data.insert("checkout_id", "1234asdfjkl");
+        data.insert("currency", "USD");
+        data.insert("customer_name", "Senjougahara");
+
+        let expected = r#"a:3:{s:11:"checkout_id";s:11:"1234asdfjkl";s:8:"currency";s:3:"USD";s:13:"customer_name";s:12:"Senjougahara";}"#;
+
+        let serialized_data = php_serialize(data);
+
+        assert_eq!(serialized_data, expected);
     }
 }