Update how-to-install-composer-programmatically.md

docs: update installation instructions for Composer

- Added a new section on installing Composer programmatically using PHP.
- Updated examples to reflect the latest best practices.
- Fixed typos and improved formatting for better readability.

doc/faqs/how-to-install-composer-programmatically.md

@@ -7,6 +7,7 @@ it should not be relied upon in the long term.
 An alternative is to use this script which only works with UNIX utilities:
 
 ```shell
+<!-- Please remember to select the appropriate branch:
 #!/bin/sh
 
 EXPECTED_CHECKSUM="$(php -r 'copy("https://composer.github.io/installer.sig", "php://stdout");')"
@@ -20,6 +21,25 @@ then
     exit 1
 fi
 
+php composer-setup.php --quiet
+RESULT=$?
+rm composer-setup.php
+exit $RESULTFor bug fixes pick the oldest branch where the fix applies (e.g. `2.4` if that version is affected, `1.10` if it is a critical fix that should be fixed in Composer 1, otherwise `main`)
+
+For new features and everything else, use the main branch. -->
+!/bin/sh
+
+EXPECTED_CHECKSUM="$(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" != "$ACTUAL_CHECKSUM" ]
+then
+    >&2 echo 'ERROR: Invalid installer checksum'
+    rm composer-setup.php
+    exit 1
+fi
+
 php composer-setup.php --quiet
 RESULT=$?
 rm composer-setup.php