Switch from php7.2 to 7.4 and from alpine to ubuntu shell script/Dockerfile question

This is in a Dockerfile with

FROM phusion/baseimage:hirsute

as foundation.

This part was working with php7.2 but we needed to switch to php7.4

The environment variable are contained in a .env file in the Format DB_USER='username'

or in the docker-compose.yml as environment as:

environment:
      - DB_USER=${DB_USER:-default} 
      - DB_PASS=password
      - DB_NAME=database

Original Part 1 of creation of dbconnect.php (Dockerfile)

RUN { \
         echo "<?php"; \
         echo "    \$db        = getenv('DB_DRIVER') ?: 'mysqli';"; \
         echo "    \$dbversion = getenv('DB_VERSION') ?: '8';"; \
         echo "    \$host      = getenv('DB_HOST') ?: 'db';"; \
         echo "    \$user      = getenv('DB_USER');"; \
         echo "    \$pass      = getenv('DB_PASS');"; \
         echo "    \$dbs       = getenv('DB_NAME') ?: 'database';"; \
         echo "    \$client_charset = 'utf8mb4';"; \ 
     } > /var/www/html/db/dbconnect.php

Original Part 1 of creation of dbconnect.php Produces in the current running container:

<?php
    $db        = getenv('DB_DRIVER') ?: 'mysqli';
    $dbversion = getenv('DB_VERSION') ?: '8';
    $host      = getenv('DB_HOST') ?: 'db';
    $user      = getenv('DB_USER');
    $pass      = getenv('DB_PASS');
    $dbs       = getenv('DB_NAME') ?: 'database';
    $client_charset = 'utf8mb4';

So password and user have no default, but anyhow it it just echos without any usage of the variables.

I played around and produced test variants.

Following Variants Part2: all not working:

 RUN { echo     "<?php"; \
       echo "\$pass=";\
       echo "${tikipass}" ; \
       echo "getenv ${DB_USER}" ; \
       echo "$DB_USER" ; \
       echo "getenv('DB_USER')"; \
       echo "hallo"; \
       echo "Count = $DB_USER.\n"; \
     } > /var/www/html/db/dbconnect.php

Output of Variant Part 2 of creation of dbconnect.php Produces in the running container:

<?php
$pass=
getenv 

getenv('DB_USER')
hallo
Count = .

So all variant i could think of just dont catch the variable content

The resulting dbconnect.php should be in the following format:

<?php
$db='mysqli';
$dbversion='8.0';
$host='localhost';
$user='user';
$pass='password';
$dbs='database';
$client_charset='utf8mb4';

Thx for the help in advance.