The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743

error.txt

Err:19 https://packages.sury.org/php stretch InRelease                         
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
Ign:26 http://ftp.au.debian.org/debian stretch InRelease                       
Hit:27 http://ftp.au.debian.org/debian stretch Release
Fetched 456 kB in 2s (189 kB/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/php stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
W: Failed to fetch https://packages.sury.org/php/dists/stretch/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
W: Some index files failed to download. They have been ignored, or old ones used instead.

fix.sh

$ sudo wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg

success.txt

--2019-05-28 14:01:05--  https://packages.sury.org/php/apt.gpg
Resolving packages.sury.org (packages.sury.org)... 104.31.95.169, 104.31.94.169, 2606:4700:30::681f:5ea9, ...
Connecting to packages.sury.org (packages.sury.org)|104.31.95.169|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1769 (1.7K) [application/octet-stream]
Saving to: ‘/etc/apt/trusted.gpg.d/php.gpg’

/etc/apt/trusted.gp 100%[===================>]   1.73K  --.-KB/s    in 0s      

2019-05-28 14:01:05 (29.1 MB/s) - ‘/etc/apt/trusted.gpg.d/php.gpg’ saved [1769/1769]

update.sh

$ sudo apt-get update

I have the same error with bookworm :

Err:7 https://packages.sury.org/php bookworm InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743

I have fix this issue with this solution :
https://packages.sury.org/php/README.txt

sudo curl -sSLo /tmp/debsuryorg-archive-keyring.deb https://packages.sury.org/debsuryorg-archive-keyring.deb
sudo dpkg -i /tmp/debsuryorg-archive-keyring.deb
sudo sh -c 'echo "deb [signed-by=/usr/share/keyrings/deb.sury.org-php.gpg] https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list'
sudo apt-get update

Thanks!
That's the only one that worked for me