For a PHAR-based Composer installation, select an exact release with composer self-update <release>. If you run the PHAR directly, use php composer.phar self-update <release>. The channel switches composer self-update --1 and composer self-update --2 select the stable 1.x or 2.x line. Before changing anything, check the PHP binary with php -v; Composer’s version requirement and your project’s PHP requirement are separate constraints.
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Choose the installation method first
The correct procedure depends on how Composer was installed. A PHAR downloaded from Composer can update itself. A Composer package supplied by your Linux distribution normally cannot use self-update, because the package manager owns the files. In that case, update or downgrade it with the distribution’s package tooling, or install a separate PHAR and call that binary explicitly.
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| Approach | Typical command | Best use | Important limitation |
|---|---|---|---|
| Exact release on an existing PHAR | composer self-update 2.4.0-RC1 |
You need one named release, including a release candidate. | Works only when the executable is a self-updating PHAR. |
| Stable major channel | composer self-update --1 or composer self-update --2 |
You need the stable Composer 1.x or 2.x line without naming a patch release. | The selected channel still has to support the PHP runtime. |
| Parallel PHAR files | php composer-setup.php --install-dir=/usr/local/bin --filename=composer2 |
Different projects or deployment jobs must use different Composer binaries. | You must call composer1 or composer2 explicitly. |
| Distribution package | Use your Linux distribution’s package command. | Your operating system must manage upgrades centrally. | self-update is unavailable for the package-managed copy. |
Exact releases and major channels are not interchangeable. An exact release is reproducible when every machine is deliberately set to that release. A channel is convenient when you want Composer to remain on the stable 1.x or 2.x track.
Check PHP compatibility before changing Composer
Run the same PHP executable that will launch Composer:
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php -vThe Composer README publishes these compatibility ranges:
| Composer line | PHP range stated by the Composer project | How to interpret it |
|---|---|---|
| Current latest Composer line | PHP 7.2.5 or newer | Use this line only when the command-line PHP runtime is at least 7.2.5. |
| Composer 2.2 LTS | PHP 5.3.2 through 8.1 | This is the compatibility line to consider for older PHP servers covered by that range. |
These are Composer’s own runtime requirements, not the requirements declared by your application. A project can require a particular PHP version even when Composer itself starts successfully. Conversely, changing Composer does not change the PHP interpreter selected by php. On a host with several PHP installations, verify the path and version before you update:
command -v php
php -v
command -v composer
composer --versionIf composer is a wrapper, symlink, or package-manager path, inspect that location before assuming it is the PHAR you intend to modify.
Prepare a safe version change
- Record the current executable and release with
command -v composerandcomposer --version. - Confirm the project uses the PHP binary you just checked.
- Decide whether every project should change or whether only one project needs another Composer binary.
- For production or CI, write the intended binary name and release into the deployment configuration instead of relying on whichever
composerappears first inPATH. - Keep a rollback path. Composer’s own data and backup directories must be writable only by their owning user; do not make them world-writable.
If the command is system-wide under /usr/local/bin or another root-owned directory, plan to run the update with the required privileges. Composer’s CLI documentation gives sudo -H composer self-update for this situation.
Change an existing PHAR to an exact release
When composer is the PHAR command
Supply the complete release identifier after self-update:
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composer self-update 2.4.0-RC12.4.0-RC1 is the documented exact-release example; replace it with the release you actually require. Composer downloads that release and replaces the PHAR used by the command.
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If the file is named composer.phar and is not on your PATH, prefix the command with PHP:
php composer.phar self-update 2.4.0-RC1Use this form whenever a shell alias or wrapper hides the executable and you want to be certain which file is changing.
When the PHAR is system-wide
A root-owned installation may reject a normal-user update. Run the same operation with the privileges needed to write the file:
sudo -H composer self-update 2.4.0-RC1The -H option gives the privileged process the appropriate home directory for Composer’s data. Afterward, verify the result as the account that will actually run your builds.
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Switch between the stable Composer 1 and 2 channels
Use a channel switch when you do not need a particular patch release:
composer self-update --1
composer self-update --2For a directly invoked PHAR, prepend php composer.phar:
php composer.phar self-update --1
php composer.phar self-update --2These commands select the stable 1.x or 2.x channel. They do not alter the PHP interpreter, your project’s declared PHP constraint, or the dependency lock file. If the selected channel cannot run on the host’s PHP version, use a compatible Composer release or upgrade PHP first.
Install two Composer versions side by side
Side-by-side binaries avoid changing a working project merely to service another one. The official installer accepts --install-dir and --filename, so two files can coexist:
- Download or obtain the installer as
composer-setup.php. - Install one binary with a distinct name:
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer2 - Install the second binary under another name:
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer1 - Select the required channel or exact release on each named binary. For example:
sudo -H /usr/local/bin/composer2 self-update --2 sudo -H /usr/local/bin/composer1 self-update --1 - For an exact release, replace the channel switch with the release identifier:
sudo -H /usr/local/bin/composer2 self-update 2.4.0-RC1 - Call the chosen name in every project or deployment script:
/usr/local/bin/composer1 install
/usr/local/bin/composer2 installDo not create two files with the same name in different directories and depend on PATH ordering. Distinct names make the choice visible in shell history, CI logs, and service definitions.
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Writing to /usr/local/bin generally requires root privileges. If you do not administer the machine, install into a user-owned directory and reference that full path instead of trying to change a system file.
Install with checksum verification in a script
For automation, verify the installer before executing it. The Composer installer FAQ documents a SHA-384 flow: fetch the expected value from https://composer.github.io/installer.sig, download the installer from https://getcomposer.org/installer, calculate hash_file('sha384', 'composer-setup.php'), and stop if the values differ.
#!/usr/bin/env bash
set -eu
EXPECTED_CHECKSUM=$(php -r "copy('https://composer.github.io/installer.sig', 'php://stdout');")
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
ACTUAL_CHECKSUM=$(php -r "echo hash_file('sha384', 'composer-setup.php');")
if [ "$EXPECTED_CHECKSUM" != "$ACTUAL_CHECKSUM" ]; then
echo 'ERROR: Invalid installer checksum' >&2
rm -f composer-setup.php
exit 1
fi
php composer-setup.php --install-dir="$HOME/bin" --filename=composer
rm -f composer-setup.phpChoose an install directory that the executing account can write. If the script installs into a root-owned directory, run only the installation step with the necessary privilege after verification, and preserve the checksum failure as a hard stop. The FAQ also describes pinning an exact installer copy to a Git commit from the Composer installer repository when reproducibility requires the installer source itself to remain fixed.
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Why composer self-update does not work
The command reports that self-update is unavailable
This usually means the executable came from a Linux distribution package rather than a PHAR. Confirm the path:
command -v composer
ls -l "$(command -v composer)"Do not overwrite a package-managed file with a PHAR. Either use the distribution’s package tooling to select the available package version, or install a PHAR under a separate name such as composer2.
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You updated one file but the shell still shows another version
Run command -v composer again and compare it with the path you updated. A shell alias, symlink, wrapper script, or earlier directory in PATH can send composer somewhere else. Invoke the intended absolute path while diagnosing:
/usr/local/bin/composer2 --version
/usr/local/bin/composer1 --versionPermission is denied
The PHAR or its containing directory is not writable by the current user. For a deliberate system-wide update, use the required privilege, such as sudo -H composer self-update --2. Do not solve the problem by making Composer’s data, backup, or binary directories writable by everyone.
The selected release will not start
Check php -v again and compare it with the Composer line’s published PHP range. Remember that the PHP binary used by an interactive shell may differ from the PHP binary used by a service, cron job, or deployment runner. Explicitly call the desired PHP executable when necessary:
/path/to/php composer.phar --version
/path/to/php composer.phar installA rollback fails or behaves unexpectedly
Composer’s rollback data and backup directories must be writable only by the owning user. An unsafe directory can permit a planted file to be installed during rollback. Correct ownership and restrictive permissions, then repeat the version change rather than broadening write access.
Verify the selected version and the project environment
- Print the release you selected:
composer --version # or php composer.phar --version - Run the project’s normal installation command:
composer installUse the explicitly named binary when versions coexist.
- Check the actual PHP and extension environment against installed package requirements when needed:
composer check-platform-reqs - Review scripts, plugins, and deployment jobs to ensure they call the intended binary name. A successful interactive test does not prove that a scheduled job uses the same executable.
composer check-platform-reqs validates the real runtime and extensions available to the installed packages. It complements, rather than replaces, the initial php -v check and the project’s own PHP constraint.
Which setup should you use?
- One temporary change: use an exact
self-update <release>on the PHAR, verify with--version, and return to the intended release afterward. - Tracking a stable generation: use
self-update --1or--2, provided the PHP runtime supports that line. - Legacy and modern projects on one server: install separate files such as
composer1andcomposer2, then reference those names in project and deployment commands. - OS-managed administration: keep using the distribution package manager, or add a separately named PHAR without replacing the managed file.
- Auditable automation: verify the installer’s SHA-384 checksum and, where necessary, pin the installer source to a known repository commit.
Whichever route you choose, the reliable sequence is: identify the binary, check PHP, select the release or channel, perform the update with appropriate permissions, and verify both Composer and the project’s platform requirements.
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