sed script with backslashes works in console or standalone script but not a Vagrantfile - bash

Question

I have this line:
sed -i 's/^\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = .*/\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = true;/' config.inc.php
Really simple line for editing my PHPMyAdmin config.inc.php AllowNoPassword to true (this is a dev environment of course).
It works perfectly in console but in a script file using vagrant it simply does not.
I do believe it is to do with the ' but I cannot understand what the difference is.
What is going on here and how to solve it?
Edit
Here is a complete example, minus a few bits of logic to simplify it and remove some private details etc:
# -*- mode: ruby -*-
# vi: set ft=ruby :
# All Vagrant configuration is done below. The "2" in Vagrant.configure
# configures the configuration version (we support older styles for
# backwards compatibility). Please don't change it unless you know what
# you're doing.
Vagrant.configure("2") do |config|
# The most common configuration options are documented and commented below.
# For a complete reference, please see the online documentation at
# https://docs.vagrantup.com.
# Every Vagrant development environment requires a box. You can search for
# boxes at https://atlas.hashicorp.com/search.
config.vm.box = "ubuntu/xenial64"
# Disable automatic box update checking. If you disable this, then
# boxes will only be checked for updates when the user runs
# `vagrant box outdated`. This is not recommended.
# config.vm.box_check_update = false
# Create a forwarded port mapping which allows access to a specific port
# within the machine from a port on the host machine. In the example below,
# accessing "localhost:8080" will access port 80 on the guest machine.
# NOTE: This will enable public access to the opened port
# config.vm.network "forwarded_port", guest: 80, host: 8080
# Create a forwarded port mapping which allows access to a specific port
# within the machine from a port on the host machine and only allow access
# via 127.0.0.1 to disable public access
config.vm.network "forwarded_port", guest: 80, host: 8080, host_ip: "127.0.0.1"
config.vm.network "forwarded_port", guest: 443, host: 4343, host_ip: "127.0.0.1"
# Create a private network, which allows host-only access to the machine
# using a specific IP.
# config.vm.network "private_network", ip: "192.168.33.10"
# Create a public network, which generally matched to bridged network.
# Bridged networks make the machine appear as another physical device on
# your network.
# config.vm.network "public_network"
# Share an additional folder to the guest VM. The first argument is
# the path on the host to the actual folder. The second argument is
# the path on the guest to mount the folder. And the optional third
# argument is a set of non-required options.
# config.vm.synced_folder "../data", "/vagrant_data"
# Provider-specific configuration so you can fine-tune various
# backing providers for Vagrant. These expose provider-specific options.
# Example for VirtualBox:
#
# config.vm.provider "virtualbox" do |vb|
# # Display the VirtualBox GUI when booting the machine
# vb.gui = true
#
# # Customize the amount of memory on the VM:
# vb.memory = "1024"
# end
#
# View the documentation for the provider you are using for more
# information on available options.
# Define a Vagrant Push strategy for pushing to Atlas. Other push strategies
# such as FTP and Heroku are also available. See the documentation at
# https://docs.vagrantup.com/v2/push/atlas.html for more information.
# config.push.define "atlas" do |push|
# push.app = "YOUR_ATLAS_USERNAME/YOUR_APPLICATION_NAME"
# end
# Enable provisioning with a shell script. Additional provisioners such as
# Puppet, Chef, Ansible, Salt, and Docker are also available. Please see the
# documentation for more information about their specific syntax and use.
config.vm.provision "shell", inline: <<-SHELL
add-apt-repository ppa:ondrej/php
apt-get update > /dev/null
yes | apt-get install zip
yes | apt-get install php7.1-fpm \
php7.1-curl \
php7.1-gd \
php7.1-mysql \
php7.1-mbstring \
php7.1-xml \
php7.1-mcrypt \
php7.1-soap \
php7.1-dev
# Set the right user for PHP7 ("ubuntu" user)
sed -i -e "s/www-data/ubuntu/g" /etc/php/7.1/fpm/pool.d/www.conf
# Fix error reporting so it is consistent with live server
sed -i 's/^error_reporting = .*/error_reporting = E_ALL/' /etc/php/7.1/fpm/php.ini
sed -i 's/^error_reporting = .*/error_reporting = E_ALL/' /etc/php/7.1/cli/php.ini
yes | apt-get install php-pear
pecl install xdebug
# Add xdebug to PHP runtime
echo 'zend_extension=xdebug.so' >> /etc/php/7.1/fpm/php.ini
echo 'zend_extension=xdebug.so' >> /etc/php/7.1/cli/php.ini
service php7.1-fpm restart
debconf-set-selections <<< 'mysql-server mysql-server/root_password password root'
debconf-set-selections <<< 'mysql-server mysql-server/root_password_again password root'
yes | apt-get -y install mysql-server
mysqladmin -u root -p'root' password ''
SHELL
$script = <<-SCRIPT
cp config.sample.inc.php config.inc.php
sed -i 's/^\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = .*/\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = true;/' config.inc.php
SCRIPT
config.vm.provision "shell", inline: $script, privileged: false
#config.vm.provision :shell, path: "bootstrap.sh"
end

Answer 1

Use an escaped heredoc:
$script = <<-'SCRIPT'
cp config.sample.inc.php config.inc.php
sed -i 's/^\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = .*/\$cfg\['\''Servers'\''\]\[\$i\]\['\''AllowNoPassword'\''\] = true;/' config.inc.php
SCRIPT
The quotes around SCRIPT indicate to the Ruby interpreter that all contents should be literal -- taken precisely as given rather than prone to expansions. (Such quotes have the same meaning in shell heredocs as well).