Can you please run ldd /usr/bin/curl to confirm that you curl binary is linked against the same libcurl.so?

The error message looks like your curl.so is borked. PHP doesn't particularly care whether your curl is linked against openssl, gnutls or nss. (We only specially handle using an old openssl version, but that should fail in a different way.)

Here you go:

linux-vdso.so.1 (0x00007ffd799e2000)
	libargon2.so.1 => /lib/x86_64-linux-gnu/libargon2.so.1 (0x00007f678a924000)
	libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f678a90a000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f678a900000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f678a77d000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f678a778000)
	libxml2.so.2 => /lib/x86_64-linux-gnu/libxml2.so.2 (0x00007f678a5cd000)
	libssl.so.1.1 => /lib/x86_64-linux-gnu/libssl.so.1.1 (0x00007f678a539000)
	libcrypto.so.1.1 => /lib/x86_64-linux-gnu/libcrypto.so.1.1 (0x00007f678a250000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f678a1cb000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f6789fad000)
	libsodium.so.23 => /lib/x86_64-linux-gnu/libsodium.so.23 (0x00007f6789f56000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6789d95000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f6789d72000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f678adda000)
	libicui18n.so.63 => /lib/x86_64-linux-gnu/libicui18n.so.63 (0x00007f6789a97000)
	libicuuc.so.63 => /lib/x86_64-linux-gnu/libicuuc.so.63 (0x00007f67898c8000)
	libicudata.so.63 => /lib/x86_64-linux-gnu/libicudata.so.63 (0x00007f6787ed8000)
	liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007f6787eb0000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f6787d2a000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f6787d10000)

Well, forget the previous one, this is it:
	linux-vdso.so.1 (0x00007fff701f9000)
	libcurl.so.4 => /lib/x86_64-linux-gnu/libcurl.so.4 (0x00007fb7049b2000)
	libplds4.so => /lib/x86_64-linux-gnu/libplds4.so (0x00007fb7049ad000)
	libnspr4.so => /lib/x86_64-linux-gnu/libnspr4.so (0x00007fb70496c000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fb70474e000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fb70472d000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb70456c000)
	libnghttp2.so.14 => /lib/x86_64-linux-gnu/libnghttp2.so.14 (0x00007fb704542000)
	libidn2.so.0 => /lib/x86_64-linux-gnu/libidn2.so.0 (0x00007fb704523000)
	libpsl.so.5 => /lib/x86_64-linux-gnu/libpsl.so.5 (0x00007fb704510000)
	libnss3.so => /lib/x86_64-linux-gnu/libnss3.so (0x00007fb7043c2000)
	libnssutil3.so => /lib/x86_64-linux-gnu/libnssutil3.so (0x00007fb704390000)
	libsmime3.so => /lib/x86_64-linux-gnu/libsmime3.so (0x00007fb704361000)
	libssl3.so => /lib/x86_64-linux-gnu/libssl3.so (0x00007fb704306000)
	libplc4.so => /lib/x86_64-linux-gnu/libplc4.so (0x00007fb7042ff000)
	libbrotlidec.so.1 => /lib/x86_64-linux-gnu/libbrotlidec.so.1 (0x00007fb7042f0000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fb7042eb000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fb7042e1000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fb704a65000)
	libunistring.so.2 => /lib/x86_64-linux-gnu/libunistring.so.2 (0x00007fb70415b000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb703fd8000)
	libbrotlicommon.so.1 => /lib/x86_64-linux-gnu/libbrotlicommon.so.1 (0x00007fb703fb5000)

Did you build PHP yourself, or are you using a pre-built PHP from
your distro/PPA?

I'm running under Debian 10. 

apt-get source curl
modified the rules to my liking, removed slack, compile with debuild, install.

Only curl got recompiled. I've tried 7.3 as it's the version provided with the distro, then also tried with the semi official Sury's php 7.4 packages and it's the same.

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Recompile PHP cURL Extension: You can try recompiling the PHP cURL extension using the libcurl version that's linked with NSS. Ensure that PHP and libcurl are both built with the same SSL backend.

Use PHP FPM: Another option is to use PHP-FPM with a separate PHP configuration. In the PHP-FPM configuration, you can specify the correct cURL library and link it to NSS instead of OpenSSL.

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I have just double checked this and there is nothing OpenSSL specific except the locking callbacks logic that was for OpenSSL pre 1.1 which is actually no longer supported so we should probably get rid of that part (that's a different thing though). Anyway there should be absolutely nothing needed for PHP to do and alternative backend should just work.

Considering that the error really shows mix up in linking library so this is really just build issue. We do not support debian builds so for that it would be better to create issue here: https://github.com/oerdnj/deb.sury.org/issues to maybe get an advice how to do it correctly.