Failed to fetch https://packages.sury.org/php/dists/stretch/InRelease

Executing in a Debian 9 an:

$ apt update

I get this error:

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.

This is because the DPA has a new signing key. To resolve this problem you have to update the APT signing key, downloading the new key from the repositories:

$ sudo wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
--2019-03-18 21:28:18--  https://packages.sury.org/php/apt.gpg
Resolving packages.sury.org (packages.sury.org)... 2001:bc8:6006:2023:aa1e:84ff:fef1:1eeb, 62.210.94.130
Connecting to packages.sury.org (packages.sury.org)|2001:bc8:6006:2023:aa1e:84ff:fef1:1eeb|: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.gpg.d/php.gpg        100%[=======================================================================>]   1.73K  --.-KB/s    in 0s

2019-03-18 21:28:18 (12.5 MB/s) - ‘/etc/apt/trusted.gpg.d/php.gpg’ saved [1769/1769]

Now the “apt update” works correctly.