[BUG] [PHP] generator ignores oneOf null branch, treating property as non-nullable

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest
  master (https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-to-test-with-the-latest-master-of-openapi-generator)
  to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (
  example (https://github.com/OpenAPITools/openapi-generator/issues/6178))

Description

The PHP generator ignores type: 'null' when it is declared inside a oneOf. As a result, the generated PHP model marks
the property as required and rejects null, even though the schema explicitly allows it. In my case, the
IntercomCompany.hosting_plan
property can be null in the API response, but the generated setter/validator throws because hosting_plan is treated as
non-nullable.

openapi-generator version

7.14.0 (docker image openapitools/openapi-generator-cli:v7.14.0)
Also reproduced with the latest version 7.16.0

OpenAPI declaration file content or url

components:
  schemas:
    HostingPricing:
      type: string
      enum:
        - launch
        - boost
        - supersonic
    IntercomCompany:
      type: object
      required:
        - channel_id
        - hosting_plan
      properties:
        channel_id:
          type: integer
          minimum: 1
        hosting_plan:
          oneOf:
            - $ref: '#/components/schemas/HostingPricing'
            - type: string
              enum:
                - enterprise
            - type: 'null'

Generation Details

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:v7.14.0 generate \
    -i /local/repro.yaml \
    -g php \
    -o /local/out

No custom config, just default PHP generator settings.

This generates the following file:

<?php
/**
 * IntercomCompany
 *
 * PHP version 8.1
 *
 * @category Class
 * @package  App\OpenApi
 * @author   OpenAPI Generator team
 * @link     https://openapi-generator.tech
 */

/**
 * Test
 *
 * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
 *
 * The version of the OpenAPI document: 1.0.0
 * Generated by: https://openapi-generator.tech
 * Generator version: 7.14.0
 */

/**
 * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * https://openapi-generator.tech
 * Do not edit the class manually.
 */

namespace App\OpenApi\Model;

use \ArrayAccess;
use \App\OpenApi\ObjectSerializer;

/**
 * IntercomCompany Class Doc Comment
 *
 * @category Class
 * @package  App\OpenApi
 * @author   OpenAPI Generator team
 * @link     https://openapi-generator.tech
 * @implements \ArrayAccess<string, mixed>
 */
class IntercomCompany implements ModelInterface, ArrayAccess, \JsonSerializable
{
    public const DISCRIMINATOR = null;

    /**
      * The original name of the model.
      *
      * @var string
      */
    protected static $openAPIModelName = 'IntercomCompany';

    /**
      * Array of property to type mappings. Used for (de)serialization
      *
      * @var string[]
      */
    protected static $openAPITypes = [
        'channel_id' => 'int',
        'hosting_plan' => '\App\OpenApi\Model\IntercomCompanyHostingPlan'
    ];

    /**
      * Array of property to format mappings. Used for (de)serialization
      *
      * @var string[]
      * @phpstan-var array<string, string|null>
      * @psalm-var array<string, string|null>
      */
    protected static $openAPIFormats = [
        'channel_id' => null,
        'hosting_plan' => null
    ];

    /**
      * Array of nullable properties. Used for (de)serialization
      *
      * @var boolean[]
      */
    protected static array $openAPINullables = [
        'channel_id' => false,
        'hosting_plan' => true
    ];

    /**
      * If a nullable field gets set to null, insert it here
      *
      * @var boolean[]
      */
    protected array $openAPINullablesSetToNull = [];

    /**
     * Array of property to type mappings. Used for (de)serialization
     *
     * @return array
     */
    public static function openAPITypes()
    {
        return self::$openAPITypes;
    }

    /**
     * Array of property to format mappings. Used for (de)serialization
     *
     * @return array
     */
    public static function openAPIFormats()
    {
        return self::$openAPIFormats;
    }

    /**
     * Array of nullable properties
     *
     * @return array
     */
    protected static function openAPINullables(): array
    {
        return self::$openAPINullables;
    }

    /**
     * Array of nullable field names deliberately set to null
     *
     * @return boolean[]
     */
    private function getOpenAPINullablesSetToNull(): array
    {
        return $this->openAPINullablesSetToNull;
    }

    /**
     * Setter - Array of nullable field names deliberately set to null
     *
     * @param boolean[] $openAPINullablesSetToNull
     */
    private function setOpenAPINullablesSetToNull(array $openAPINullablesSetToNull): void
    {
        $this->openAPINullablesSetToNull = $openAPINullablesSetToNull;
    }

    /**
     * Checks if a property is nullable
     *
     * @param string $property
     * @return bool
     */
    public static function isNullable(string $property): bool
    {
        return self::openAPINullables()[$property] ?? false;
    }

    /**
     * Checks if a nullable property is set to null.
     *
     * @param string $property
     * @return bool
     */
    public function isNullableSetToNull(string $property): bool
    {
        return in_array($property, $this->getOpenAPINullablesSetToNull(), true);
    }

    /**
     * Array of attributes where the key is the local name,
     * and the value is the original name
     *
     * @var string[]
     */
    protected static $attributeMap = [
        'channel_id' => 'channel_id',
        'hosting_plan' => 'hosting_plan'
    ];

    /**
     * Array of attributes to setter functions (for deserialization of responses)
     *
     * @var string[]
     */
    protected static $setters = [
        'channel_id' => 'setChannelId',
        'hosting_plan' => 'setHostingPlan'
    ];

    /**
     * Array of attributes to getter functions (for serialization of requests)
     *
     * @var string[]
     */
    protected static $getters = [
        'channel_id' => 'getChannelId',
        'hosting_plan' => 'getHostingPlan'
    ];

    /**
     * Array of attributes where the key is the local name,
     * and the value is the original name
     *
     * @return array
     */
    public static function attributeMap()
    {
        return self::$attributeMap;
    }

    /**
     * Array of attributes to setter functions (for deserialization of responses)
     *
     * @return array
     */
    public static function setters()
    {
        return self::$setters;
    }

    /**
     * Array of attributes to getter functions (for serialization of requests)
     *
     * @return array
     */
    public static function getters()
    {
        return self::$getters;
    }

    /**
     * The original name of the model.
     *
     * @return string
     */
    public function getModelName()
    {
        return self::$openAPIModelName;
    }


    /**
     * Associative array for storing property values
     *
     * @var mixed[]
     */
    protected $container = [];

    /**
     * Constructor
     *
     * @param mixed[]|null $data Associated array of property values
     *                      initializing the model
     */
    public function __construct(?array $data = null)
    {
        $this->setIfExists('channel_id', $data ?? [], null);
        $this->setIfExists('hosting_plan', $data ?? [], null);
    }

    /**
    * Sets $this->container[$variableName] to the given data or to the given default Value; if $variableName
    * is nullable and its value is set to null in the $fields array, then mark it as "set to null" in the
    * $this->openAPINullablesSetToNull array
    *
    * @param string $variableName
    * @param array  $fields
    * @param mixed  $defaultValue
    */
    private function setIfExists(string $variableName, array $fields, $defaultValue): void
    {
        if (self::isNullable($variableName) && array_key_exists($variableName, $fields) && is_null($fields[$variableName])) {
            $this->openAPINullablesSetToNull[] = $variableName;
        }

        $this->container[$variableName] = $fields[$variableName] ?? $defaultValue;
    }

    /**
     * Show all the invalid properties with reasons.
     *
     * @return array invalid properties with reasons
     */
    public function listInvalidProperties()
    {
        $invalidProperties = [];

        if ($this->container['channel_id'] === null) {
            $invalidProperties[] = "'channel_id' can't be null";
        }
        if (($this->container['channel_id'] < 1)) {
            $invalidProperties[] = "invalid value for 'channel_id', must be bigger than or equal to 1.";
        }

        if ($this->container['hosting_plan'] === null) {
            $invalidProperties[] = "'hosting_plan' can't be null";
        }
        return $invalidProperties;
    }

    /**
     * Validate all the properties in the model
     * return true if all passed
     *
     * @return bool True if all properties are valid
     */
    public function valid()
    {
        return count($this->listInvalidProperties()) === 0;
    }


    /**
     * Gets channel_id
     *
     * @return int
     */
    public function getChannelId()
    {
        return $this->container['channel_id'];
    }

    /**
     * Sets channel_id
     *
     * @param int $channel_id channel_id
     *
     * @return self
     */
    public function setChannelId($channel_id)
    {
        if (is_null($channel_id)) {
            throw new \InvalidArgumentException('non-nullable channel_id cannot be null');
        }

        if (($channel_id < 1)) {
            throw new \InvalidArgumentException('invalid value for $channel_id when calling IntercomCompany., must be bigger than or equal to 1.');
        }

        $this->container['channel_id'] = $channel_id;

        return $this;
    }

    /**
     * Gets hosting_plan
     *
     * @return \App\OpenApi\Model\IntercomCompanyHostingPlan
     */
    public function getHostingPlan()
    {
        return $this->container['hosting_plan'];
    }

    /**
     * Sets hosting_plan
     *
     * @param \App\OpenApi\Model\IntercomCompanyHostingPlan $hosting_plan hosting_plan
     *
     * @return self
     */
    public function setHostingPlan($hosting_plan)
    {
        if (is_null($hosting_plan)) {
            array_push($this->openAPINullablesSetToNull, 'hosting_plan');
        } else {
            $nullablesSetToNull = $this->getOpenAPINullablesSetToNull();
            $index = array_search('hosting_plan', $nullablesSetToNull);
            if ($index !== FALSE) {
                unset($nullablesSetToNull[$index]);
                $this->setOpenAPINullablesSetToNull($nullablesSetToNull);
            }
        }
        $this->container['hosting_plan'] = $hosting_plan;

        return $this;
    }
    /**
     * Returns true if offset exists. False otherwise.
     *
     * @param integer $offset Offset
     *
     * @return boolean
     */
    public function offsetExists($offset): bool
    {
        return isset($this->container[$offset]);
    }

    /**
     * Gets offset.
     *
     * @param integer $offset Offset
     *
     * @return mixed|null
     */
    #[\ReturnTypeWillChange]
    public function offsetGet($offset)
    {
        return $this->container[$offset] ?? null;
    }

    /**
     * Sets value based on offset.
     *
     * @param int|null $offset Offset
     * @param mixed    $value  Value to be set
     *
     * @return void
     */
    public function offsetSet($offset, $value): void
    {
        if (is_null($offset)) {
            $this->container[] = $value;
        } else {
            $this->container[$offset] = $value;
        }
    }

    /**
     * Unsets offset.
     *
     * @param integer $offset Offset
     *
     * @return void
     */
    public function offsetUnset($offset): void
    {
        unset($this->container[$offset]);
    }

    /**
     * Serializes the object to a value that can be serialized natively by json_encode().
     * @link https://www.php.net/manual/en/jsonserializable.jsonserialize.php
     *
     * @return mixed Returns data which can be serialized by json_encode(), which is a value
     * of any type other than a resource.
     */
    #[\ReturnTypeWillChange]
    public function jsonSerialize()
    {
       return ObjectSerializer::sanitizeForSerialization($this);
    }

    /**
     * Gets the string presentation of the object
     *
     * @return string
     */
    public function __toString()
    {
        return json_encode(
            ObjectSerializer::sanitizeForSerialization($this),
            JSON_PRETTY_PRINT
        );
    }

    /**
     * Gets a header-safe presentation of the object
     *
     * @return string
     */
    public function toHeaderValue()
    {
        return json_encode(ObjectSerializer::sanitizeForSerialization($this));
    }
}

Which contains:

        if ($this->container['hosting_plan'] === null) {
            $invalidProperties[] = "'hosting_plan' can't be null";
        }

Steps to reproduce

1. Use the spec above (repro.yaml) and run the generation command shown.
2. Inspect the generated Model/IntercomCompany.php.
3. Note that listInvalidProperties() contains "'hosting_plan' can't be null" and the setter throws when null is given.
4. Try to instantiate the model with ['channel_id' => 1, 'hosting_plan' => null] and call valid(). It returns false
   because hosting_plan is treated as non-nullable.

Related issues/PRs

I couldn’t find an existing issue covering oneOf with type: 'null' in the PHP generator.

Suggest a fix

When a schema contains a oneOf (or anyOf) branch with type: 'null', the generator should mark the property as nullable
in the PHP model (openAPINullables['hosting_plan'] = true) and skip the 'hosting_plan' can't be null validation. The
same applies to
other generators that rely on similar logic.

Start by running the provided Docker command with repro.yaml and inspect the generated IntercomCompany model, especially hosting_plan validation and setter behavior. Trace the PHP generator's handling of oneOf branches containing type: 'null'. Done means a required hosting_plan accepts null when the schema includes that branch, without rejecting valid API responses.