Our Backward Compatibility Promise

Ensuring smooth upgrades of your projects is our first priority. That's why we promise you backward compatibility (BC) for all minor Symfony releases. You probably recognize this strategy as Semantic Versioning. In short, Semantic Versioning means that only major releases (such as 5.0, 6.0 etc.) are allowed to break backward compatibility. Minor releases (such as 5.1, 5.2 etc.) may introduce new features, but must do so without breaking the existing API of that release branch (5.x in the previous example).

We also provide deprecation message triggered in the code base to help you with the migration process across major releases.

Caution

This promise was introduced with Symfony 2.3 and does not apply to previous versions of Symfony.

However, backward compatibility comes in many different flavors. In fact, almost every change that we make to the framework can potentially break an application. For example, if we add a new method to a class, this will break an application which extended this class and added the same method, but with a different method signature.

Also, not every BC break has the same impact on application code. While some BC breaks require you to make significant changes to your classes or your architecture, others are fixed by changing the name of a method.

That's why we created this page for you. The section "Using Symfony Code" will tell you how you can ensure that your application won't break completely when upgrading to a newer version of the same major release branch.

The second section, "Working on Symfony Code", is targeted at Symfony contributors. This section lists detailed rules that every contributor needs to follow to ensure smooth upgrades for our users.

Caution

Experimental Features and code marked with the @internal tags are excluded from our Backward Compatibility promise.

Also note that backward compatibility breaks are tolerated if they are required to fix a security issue.

Using Symfony Code

If you are using Symfony in your projects, the following guidelines will help you to ensure smooth upgrades to all future minor releases of your Symfony version.

Using our Interfaces

All interfaces shipped with Symfony can be used in type hints. You can also call any of the methods that they declare. We guarantee that we won't break code that sticks to these rules.

Caution

The exception to this rule are interfaces tagged with @internal. Such interfaces should not be used or implemented.

If you implement an interface, we promise that we won't ever break your code.

The following table explains in detail which use cases are covered by our backward compatibility promise:

Use Case	Backward Compatibility
If you...	Then we guarantee BC...
Type hint against the interface	Yes
Call a method	Yes [10]
If you implement the interface and...	Then we guarantee BC...
Implement a method	Yes
Add an argument to an implemented method	Yes
Add a default value to an argument	Yes
Add a return type to an implemented method	Yes

Using our Classes

All classes provided by Symfony may be instantiated and accessed through their public methods and properties.

Caution

Classes, properties and methods that bear the tag @internal as well as the classes located in the various *\Tests\ namespaces are an exception to this rule. They are meant for internal use only and should not be accessed by your own code.

To be on the safe side, check the following table to know which use cases are covered by our backward compatibility promise:

Use Case	Backward Compatibility
If you...	Then we guarantee BC...
Type hint against the class	Yes
Create a new instance	Yes
Extend the class	Yes
Access a public property	Yes
Call a public method	Yes [10]
If you extend the class and...	Then we guarantee BC...
Access a protected property	Yes
Call a protected method	Yes [10]
Override a public property	Yes
Override a protected property	Yes
Override a public method	Yes
Override a protected method	Yes
Add a new property	No
Add a new method	No
Add an argument to an overridden method	Yes
Add a default value to an argument	Yes
Call a private method (via Reflection)	No
Access a private property (via Reflection)	No

Using our Traits

All traits provided by Symfony may be used in your classes.

Caution

The exception to this rule are traits tagged with @internal. Such traits should not be used.

To be on the safe side, check the following table to know which use cases are covered by our backward compatibility promise:

Use Case	Backward Compatibility
If you...	Then we guarantee BC...
Use a trait	Yes
If you use the trait and...	Then we guarantee BC...
Use it to implement an interface	Yes
Use it to implement an abstract method	Yes
Use it to extend a parent class	Yes
Use it to define an abstract class	Yes
Use a public, protected or private property	Yes
Use a public, protected or private method	Yes

Working on Symfony Code

Do you want to help us improve Symfony? That's great! However, please stick to the rules listed below in order to ensure smooth upgrades for our users.

Changing Interfaces

This table tells you which changes you are allowed to do when working on Symfony's interfaces:

Type of Change	Change Allowed	Notes
Remove entirely	No
Change name or namespace	No
Add parent interface	Yes	[2]
Remove parent interface	No
Methods
Add method	No
Remove method	No
Change name	No
Move to parent interface	Yes
Add argument without a default value	No
Add argument with a default value	No
Remove argument	No	[3]
Add default value to an argument	No
Remove default value of an argument	No
Add type hint to an argument	No
Remove type hint of an argument	No
Change argument type	No
Add return type	No
Remove return type	No	[9]
Change return type	No
Static Methods
Turn non static into static	No
Turn static into non static	No
Constants
Add constant	Yes
Remove constant	No
Change value of a constant	Yes	[1] [5]

Changing Classes

This table tells you which changes you are allowed to do when working on Symfony's classes:

Type of Change	Change Allowed	Notes
Remove entirely	No
Make final	No	[6]
Make abstract	No
Change name or namespace	No
Change parent class	Yes	[4]
Add interface	Yes
Remove interface	No
Public Properties
Add public property	Yes
Remove public property	No
Reduce visibility	No
Move to parent class	Yes
Protected Properties
Add protected property	Yes
Remove protected property	No	[7]
Reduce visibility	No	[7]
Make public	No	[7]
Move to parent class	Yes
Private Properties
Add private property	Yes
Make public or protected	Yes
Remove private property	Yes
Constructors
Add constructor without mandatory arguments	Yes	[1]
Add argument without a default value	No
Add argument with a default value	Yes	[11]
Remove argument	No	[3]
Add default value to an argument	Yes
Remove default value of an argument	No
Add type hint to an argument	No
Remove type hint of an argument	Yes
Change argument type	No
Remove constructor	No
Reduce visibility of a public constructor	No
Reduce visibility of a protected constructor	No	[7]
Move to parent class	Yes
Destructors
Add destructor	Yes
Remove destructor	No
Move to parent class	Yes
Public Methods
Add public method	Yes
Remove public method	No
Change name	No
Reduce visibility	No
Make final	No	[6]
Move to parent class	Yes
Add argument without a default value	No
Add argument with a default value	No	[7] [8]
Remove argument	No	[3]
Add default value to an argument	No	[7] [8]
Remove default value of an argument	No
Add type hint to an argument	No	[7] [8]
Remove type hint of an argument	No	[7] [8]
Change argument type	No	[7] [8]
Add return type	No	[7] [8]
Remove return type	No	[7] [8] [9]
Change return type	No	[7] [8]
Protected Methods
Add protected method	Yes
Remove protected method	No	[7]
Change name	No	[7]
Reduce visibility	No	[7]
Make final	No	[6]
Make public	No	[7] [8]
Move to parent class	Yes
Add argument without a default value	No
Add argument with a default value	No	[7] [8]
Remove argument	No	[3]
Add default value to an argument	No	[7] [8]
Remove default value of an argument	No	[7]
Add type hint to an argument	No	[7] [8]
Remove type hint of an argument	No	[7] [8]
Change argument type	No	[7] [8]
Add return type	No	[7] [8]
Remove return type	No	[7] [8] [9]
Change return type	No	[7] [8]
Private Methods
Add private method	Yes
Remove private method	Yes
Change name	Yes
Make public or protected	Yes
Add argument without a default value	Yes
Add argument with a default value	Yes
Remove argument	Yes
Add default value to an argument	Yes
Remove default value of an argument	Yes
Add type hint to an argument	Yes
Remove type hint of an argument	Yes
Change argument type	Yes
Add return type	Yes
Remove return type	Yes
Change return type	Yes
Static Methods and Properties
Turn non static into static	No	[7] [8]
Turn static into non static	No
Constants
Add constant	Yes
Remove constant	No
Change value of a constant	Yes	[1] [5]

Changing Traits

This table tells you which changes you are allowed to do when working on Symfony's traits:

Type of Change	Change Allowed	Notes
Remove entirely	No
Change name or namespace	No
Use another trait	Yes
Public Properties
Add public property	Yes
Remove public property	No
Reduce visibility	No
Move to a used trait	Yes
Protected Properties
Add protected property	Yes
Remove protected property	No
Reduce visibility	No
Make public	No
Move to a used trait	Yes
Private Properties
Add private property	Yes
Remove private property	No
Make public or protected	Yes
Move to a used trait	Yes
Constructors and destructors
Have constructor or destructor	No
Public Methods
Add public method	Yes
Remove public method	No
Change name	No
Reduce visibility	No
Make final	No	[6]
Move to used trait	Yes
Add argument without a default value	No
Add argument with a default value	No
Remove argument	No
Add default value to an argument	No
Remove default value of an argument	No
Add type hint to an argument	No
Remove type hint of an argument	No
Change argument type	No
Change return type	No
Protected Methods
Add protected method	Yes
Remove protected method	No
Change name	No
Reduce visibility	No
Make final	No	[6]
Make public	No	[8]
Move to used trait	Yes
Add argument without a default value	No
Add argument with a default value	No
Remove argument	No
Add default value to an argument	No
Remove default value of an argument	No
Add type hint to an argument	No
Remove type hint of an argument	No
Change argument type	No
Change return type	No
Private Methods
Add private method	Yes
Remove private method	No
Change name	No
Make public or protected	Yes
Move to used trait	Yes
Add argument without a default value	No
Add argument with a default value	No
Remove argument	No
Add default value to an argument	No
Remove default value of an argument	No
Add type hint to an argument	No
Remove type hint of an argument	No
Change argument type	No
Add return type	No
Remove return type	No
Change return type	No
Static Methods and Properties
Turn non static into static	No
Turn static into non static	No

Notes

[1] Should be avoided. When done, this change must be documented in the UPGRADE file.

[2] The added parent interface must not introduce any new methods that don't exist in the interface already.

[3] Only the last optional argument(s) of a method may be removed, as PHP does not care about additional arguments that you pass to a method.

[4] When changing the parent class, the original parent class must remain an ancestor of the class.

[5] The value of a constant may only be changed when the constants aren't used in configuration (e.g. Yaml and XML files), as these do not support constants and have to hardcode the value. For instance, event name constants can't change the value without introducing a BC break. Additionally, if a constant will likely be used in objects that are serialized, the value of a constant should not be changed.

[6] Allowed using the @final annotation.

[7] Allowed if the class is final. Classes that received the @final annotation after their first release are considered final in their next major version. Changing an argument type is only possible with a parent type. Changing a return type is only possible with a child type.

[8] Allowed if the method is final. Methods that received the @final annotation after their first release are considered final in their next major version. Changing an argument type is only possible with a parent type. Changing a return type is only possible with a child type.

[9] Allowed for the void return type.

[10] Parameter names are only covered by the compatibility promise for constructors of Attribute classes. Using PHP named arguments might break your code when upgrading to newer Symfony versions.

[11] Only optional argument(s) of a constructor at last position may be added.

Making Code Changes in a Backward Compatible Way

As you read above, many changes are not allowed because they would represent a backward compatibility break. However, we want to be able to improve the code and its features over time and that can be done thanks to some strategies that allow to still do some unallowed changes in several steps that ensure backward compatibility and a smooth upgrade path. Some of them are described in the next sections.

Adding an Argument to a Public Method

Adding a new argument to a public method is possible only if this is the last argument of the method.

If that's the case, here is how to do it properly in a minor version:

Add the argument as a comment in the signature:

        1
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8
9
        // the new argument can be optional
public function say(string $text, /* bool $stripWhitespace = true */): void
{
}

// or required
public function say(string $text, /* bool $stripWhitespace */): void
{
}
    

Document the new argument in a PHPDoc:

        1
2
3
        /**
 * @param bool $stripWhitespace
 */
    

Use func_num_args and func_get_arg to retrieve the argument in the method:
```
1
```
```
$stripWhitespace = 2 <= \func_num_args() ? func_get_arg(1) : false;
```
Note that the default value is false to keep the current behavior.

If the argument has a default value that will change the current behavior, warn the user:

        1
        trigger_deprecation('symfony/COMPONENT', 'X.Y', 'Not passing the "bool $stripWhitespace" argument explicitly is deprecated, its default value will change to X in Z.0.');
    

If the argument has no default value, warn the user that is going to be required in the next major version:

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        if (\func_num_args() < 2) {
    trigger_deprecation('symfony/COMPONENT', 'X.Y', 'The "%s()" method will have a new "bool $stripWhitespace" argument in version Z.0, not defining it is deprecated.', __METHOD__);

    $stripWhitespace = false;
} else {
    $stripWhitespace = func_get_arg(1);
}
    

In the next major version (X.0), uncomment the argument, remove the PHPDoc if there is no need for a description, and remove the func_get_arg code and the warning if any.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.

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