diff --git a/src/Internal/functions.php b/src/Internal/functions.php
index f51e643..35320a5 100644
--- a/src/Internal/functions.php
+++ b/src/Internal/functions.php
@@ -2,8 +2,6 @@
 
 namespace Amp\Postgres\Internal;
 
-use function Amp\Postgres\cast;
-
 /** @internal */
 const STATEMENT_PARAM_REGEX = <<<'REGEX'
 [
@@ -92,3 +90,53 @@ function replaceNamedParams(array $params, array $names): array
 
     return $values;
 }
+
+/**
+ * @internal
+ *
+ * Casts a PHP value to a representation that is understood by Postgres, including encoding arrays.
+ *
+ * @throws \Error If $value is an object which is not a BackedEnum or Stringable, a resource, or an unknown type.
+ */
+function cast(mixed $value): string|int|float|null
+{
+    return match (\gettype($value)) {
+        "NULL", "integer", "double", "string" => $value,
+        "boolean" => $value ? 't' : 'f',
+        "array" => '{' . \implode(',', \array_map(encodeArrayItem(...), $value)) . '}',
+        "object" => match (true) {
+            $value instanceof \BackedEnum => $value->value,
+            $value instanceof \Stringable => (string) $value,
+            default => throw new \ValueError(
+                "An object in parameter values must be a BackedEnum or implement Stringable; got instance of "
+                . \get_debug_type($value)
+            ),
+        },
+        default => throw new \ValueError(\sprintf(
+            "Invalid value type '%s' in parameter values",
+            \get_debug_type($value),
+        )),
+    };
+}
+
+/**
+ * @internal
+ *
+ * Wraps string in double-quotes for inclusion in an array.
+ */
+function encodeArrayItem(mixed $value): mixed
+{
+    return match (\gettype($value)) {
+        "NULL" => "NULL",
+        "string" => '"' . \str_replace(['\\', '"'], ['\\\\', '\\"'], $value) . '"',
+        "object" => match (true) {
+            $value instanceof \BackedEnum => encodeArrayItem($value->value),
+            $value instanceof \Stringable => encodeArrayItem((string) $value),
+            default => throw new \ValueError(
+                "An object in parameter arrays must be a BackedEnum or implement Stringable; "
+                . "got instance of " . \get_debug_type($value)
+            ),
+        },
+        default => cast($value),
+    };
+}
diff --git a/src/functions.php b/src/functions.php
index 5f4a803..c1c7886 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -41,68 +41,3 @@ function connect(PostgresConfig $config, ?Cancellation $cancellation = null): Po
 {
     return postgresConnector()->connect($config, $cancellation);
 }
-
-/**
- * Casts a PHP value to a representation that is understood by Postgres, including encoding arrays.
- *
- * @throws \Error If $value is an object without a __toString() method, a resource, or an unknown type.
- */
-function cast(mixed $value): string|int|float|null
-{
-    switch ($type = \gettype($value)) {
-        case "NULL":
-        case "integer":
-        case "double":
-        case "string":
-            return $value; // No casting necessary for numerics, strings, and null.
-
-        case "boolean":
-            return $value ? 't' : 'f';
-
-        case "array":
-            return encode($value);
-
-        case "object":
-            if (!\method_exists($value, "__toString")) {
-                throw new \Error("Object without a __toString() method included in parameter values");
-            }
-
-            return (string) $value;
-
-        default:
-            throw new \Error("Invalid value type '$type' in parameter values");
-    }
-}
-
-/**
- * Encodes an array into a PostgreSQL representation of the array.
- *
- * @return string The serialized representation of the array.
- *
- * @throws \Error If $array contains an object without a __toString() method, a resource, or an unknown type.
- */
-function encode(array $array): string
-{
-    $array = \array_map(function ($value) {
-        switch (\gettype($value)) {
-            case "NULL":
-                return "NULL";
-
-            case "object":
-                if (!\method_exists($value, "__toString")) {
-                    throw new \Error("Object without a __toString() method in array");
-                }
-
-                $value = (string) $value;
-                // no break
-
-            case "string":
-                return '"' . \str_replace(['\\', '"'], ['\\\\', '\\"'], $value) . '"';
-
-            default:
-                return cast($value); // Recursively encodes arrays and errors on invalid values.
-        }
-    }, $array);
-
-    return '{' . \implode(',', $array) . '}';
-}
diff --git a/test/CastTest.php b/test/CastTest.php
new file mode 100644
index 0000000..dca1d0a
--- /dev/null
+++ b/test/CastTest.php
@@ -0,0 +1,149 @@
+<?php declare(strict_types=1);
+
+namespace Amp\Postgres\Test;
+
+use PHPUnit\Framework\TestCase;
+use function Amp\Postgres\Internal\cast;
+
+enum IntegerEnum: int
+{
+    case One = 1;
+    case Two = 2;
+    case Three = 3;
+}
+
+enum StringEnum: string
+{
+    case One = 'one';
+    case Two = 'two';
+    case Three = 'three';
+}
+
+enum UnitEnum
+{
+    case Case;
+}
+
+class CastTest extends TestCase
+{
+    public function testSingleDimensionalStringArray(): void
+    {
+        $array = ["one", "two", "three"];
+        $string = '{"one","two","three"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testMultiDimensionalStringArray(): void
+    {
+        $array = ["one", "two", ["three", "four"], "five"];
+        $string = '{"one","two",{"three","four"},"five"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testQuotedStrings(): void
+    {
+        $array = ["one", "two", ["three", "four"], "five"];
+        $string = '{"one","two",{"three","four"},"five"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testEscapedQuoteDelimiter(): void
+    {
+        $array = ['va"lue1', 'value"2'];
+        $string = '{"va\\"lue1","value\\"2"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testNullValue(): void
+    {
+        $array = ["one", null, "three"];
+        $string = '{"one",NULL,"three"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testSingleDimensionalIntegerArray(): void
+    {
+        $array = [1, 2, 3];
+        $string = '{' . \implode(',', $array) . '}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testIntegerArrayWithNull(): void
+    {
+        $array = [1, 2, null, 3];
+        $string = '{1,2,NULL,3}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testMultidimensionalIntegerArray(): void
+    {
+        $array = [1, 2, [3, 4], [5], 6, 7, [[8, 9], 10]];
+        $string = '{1,2,{3,4},{5},6,7,{{8,9},10}}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testEscapedBackslashesInQuotedValue(): void
+    {
+        $array = ["test\\ing", "esca\\ped\\"];
+        $string = '{"test\\\\ing","esca\\\\ped\\\\"}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testBackedEnum(): void
+    {
+        $this->assertSame(3, cast(IntegerEnum::Three));
+        $this->assertSame('three', cast(StringEnum::Three));
+    }
+
+    public function testBackedEnumInArray(): void
+    {
+        $array = [
+            [IntegerEnum::One, IntegerEnum::Two, IntegerEnum::Three],
+            [StringEnum::One, StringEnum::Two, StringEnum::Three],
+        ];
+        $string = '{{1,2,3},{"one","two","three"}}';
+
+        $this->assertSame($string, cast($array));
+    }
+
+    public function testUnitEnum(): void
+    {
+        $this->expectException(\ValueError::class);
+        $this->expectExceptionMessage('An object in parameter values must be');
+
+        cast(UnitEnum::Case);
+    }
+
+    public function testUnitEnumInArray(): void
+    {
+        $this->expectException(\ValueError::class);
+        $this->expectExceptionMessage('An object in parameter arrays must be');
+
+        cast([UnitEnum::Case]);
+    }
+
+    public function testObjectWithoutToStringMethod(): void
+    {
+        $this->expectException(\ValueError::class);
+        $this->expectExceptionMessage('An object in parameter values must be');
+
+        cast(new \stdClass);
+    }
+
+    public function testObjectWithoutToStringMethodInArray(): void
+    {
+        $this->expectException(\ValueError::class);
+        $this->expectExceptionMessage('An object in parameter arrays must be');
+
+        cast([new \stdClass]);
+    }
+}
diff --git a/test/EncodeTest.php b/test/EncodeTest.php
deleted file mode 100644
index 16dd442..0000000
--- a/test/EncodeTest.php
+++ /dev/null
@@ -1,89 +0,0 @@
-<?php declare(strict_types=1);
-
-namespace Amp\Postgres\Test;
-
-use PHPUnit\Framework\TestCase;
-use function Amp\Postgres\encode;
-
-class EncodeTest extends TestCase
-{
-    public function testSingleDimensionalStringArray(): void
-    {
-        $array = ["one", "two", "three"];
-        $string = '{"one","two","three"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testMultiDimensionalStringArray(): void
-    {
-        $array = ["one", "two", ["three", "four"], "five"];
-        $string = '{"one","two",{"three","four"},"five"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testQuotedStrings(): void
-    {
-        $array = ["one", "two", ["three", "four"], "five"];
-        $string = '{"one","two",{"three","four"},"five"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testEscapedQuoteDelimiter(): void
-    {
-        $array = ['va"lue1', 'value"2'];
-        $string = '{"va\\"lue1","value\\"2"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testNullValue(): void
-    {
-        $array = ["one", null, "three"];
-        $string = '{"one",NULL,"three"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testSingleDimensionalIntegerArray(): void
-    {
-        $array = [1, 2, 3];
-        $string = '{' . \implode(',', $array) . '}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testIntegerArrayWithNull(): void
-    {
-        $array = [1, 2, null, 3];
-        $string = '{1,2,NULL,3}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testMultidimensionalIntegerArray(): void
-    {
-        $array = [1, 2, [3, 4], [5], 6, 7, [[8, 9], 10]];
-        $string = '{1,2,{3,4},{5},6,7,{{8,9},10}}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testEscapedBackslashesInQuotedValue(): void
-    {
-        $array = ["test\\ing", "esca\\ped\\"];
-        $string = '{"test\\\\ing","esca\\\\ped\\\\"}';
-
-        $this->assertSame($string, encode($array));
-    }
-
-    public function testObjectWithoutToStringMethod(): void
-    {
-        $this->expectException(\Error::class);
-        $this->expectExceptionMessage('Object without a __toString() method in array');
-
-        encode([new \stdClass]);
-    }
-}
diff --git a/test/PgSqlConnectionTest.php b/test/PgSqlConnectionTest.php
index d27dfe5..a01c5e4 100644
--- a/test/PgSqlConnectionTest.php
+++ b/test/PgSqlConnectionTest.php
@@ -5,7 +5,7 @@
 use Amp\Postgres\PgSqlConnection;
 use Amp\Postgres\PostgresLink;
 use Revolt\EventLoop;
-use function Amp\Postgres\cast;
+use function Amp\Postgres\Internal\cast;
 
 /**
  * @requires extension pgsql
diff --git a/test/PgSqlPoolTest.php b/test/PgSqlPoolTest.php
index f99dcd4..baa02dc 100644
--- a/test/PgSqlPoolTest.php
+++ b/test/PgSqlPoolTest.php
@@ -9,7 +9,7 @@
 use Amp\Sql\Common\ConnectionPool;
 use Amp\Sql\SqlConnector;
 use Revolt\EventLoop;
-use function Amp\Postgres\cast;
+use function Amp\Postgres\Internal\cast;
 
 /**
  * @requires extension pgsql
diff --git a/test/PqConnectionTest.php b/test/PqConnectionTest.php
index 73a5525..0634f88 100644
--- a/test/PqConnectionTest.php
+++ b/test/PqConnectionTest.php
@@ -6,7 +6,7 @@
 use Amp\Postgres\Internal\PqUnbufferedResultSet;
 use Amp\Postgres\PostgresLink;
 use Amp\Postgres\PqConnection;
-use function Amp\Postgres\cast;
+use function Amp\Postgres\Internal\cast;
 
 /**
  * @requires extension pq
diff --git a/test/PqPoolTest.php b/test/PqPoolTest.php
index 568d8f9..e25fa7c 100644
--- a/test/PqPoolTest.php
+++ b/test/PqPoolTest.php
@@ -8,7 +8,7 @@
 use Amp\Postgres\PqConnection;
 use Amp\Sql\Common\ConnectionPool;
 use Amp\Sql\SqlConnector;
-use function Amp\Postgres\cast;
+use function Amp\Postgres\Internal\cast;
 
 /**
  * @requires extension pq