参考思路：https://unix.stackexchange.com/questions/389156/how-to-fix-held-broken-packages
安装php7.4-dev的时候，提示“Depends: libssl-dev but it is not going to be installed”。
又试着安装 libssl-dev，提示“Depends: libssl1.1 (= 1.1.1f-1ubuntu2.2) but 1.1.1j-1+ubuntu20.04.1+deb.sury.org+3 is to be installed”。
根据[参考思路]链接，尝试：
sudo apt update
sudo apt install php-common
更新下 php-common 的安装版本，再安装 php7.4-dev 就可以正确安装了。
$ sudo apt install php7.3-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
autoconf automake autopoint autotools-dev debhelper dh-autoreconf dh-strip-nondeterminism
dwz gettext intltool-debian libarchive-cpio-perl libarchive-zip-perl libcroco3
libdebhelper-perl libfile-stripnondeterminism-perl libltdl-dev libmail-sendmail-perl
libpcre2-dev libpcre2-posix2 libsigsegv2 libssl-dev libssl1.1 libsub-override-perl
libsys-hostname-long-perl libtool m4 pkg-php-tools po-debconf shtool
Suggested packages:
autoconf-archive gnu-standards autoconf-doc dh-make gettext-doc libasprintf-dev
libgettextpo-dev libtool-doc libssl-doc gfortran | fortran95-compiler gcj-jdk m4-doc dh-php
libmail-box-perl
The following NEW packages will be installed:
autoconf automake autopoint autotools-dev debhelper dh-autoreconf dh-strip-nondeterminism
dwz gettext intltool-debian libarchive-cpio-perl libarchive-zip-perl libcroco3
libdebhelper-perl libfile-stripnondeterminism-perl libltdl-dev libmail-sendmail-perl
libpcre2-dev libpcre2-posix2 libsigsegv2 libssl-dev libsub-override-perl
libsys-hostname-long-perl libtool m4 php7.3-dev pkg-php-tools po-debconf shtool
The following packages will be DOWNGRADED:
libssl1.1
0 upgraded, 29 newly installed, 1 downgraded, 0 to remove and 54 not upgraded.
Need to get 2,901 kB/8,517 kB of archives.
After this operation, 30.7 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates/main amd64 libssl1.1 amd64 1.1.1f-1ubuntu2.2 [1,319 kB]
Get:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates/main amd64 libssl-dev amd64 1.1.1f-1ubuntu2.2 [1,582 kB]
Fetched 2,901 kB in 4s (753 kB/s)
Preconfiguring packages ...
dpkg: warning: downgrading libssl1.1:amd64 from 1.1.1j-1+ubuntu20.04.1+deb.sury.org+3 to 1.1.1f-1ubuntu2.2
(Reading database ... 214645 files and directories currently installed.)
Preparing to unpack .../00-libssl1.1_1.1.1f-1ubuntu2.2_amd64.deb ...
Unpacking libssl1.1:amd64 (1.1.1f-1ubuntu2.2) over (1.1.1j-1+ubuntu20.04.1+deb.sury.org+3) ...
Selecting previously unselected package libsigsegv2:amd64.
Preparing to unpack .../01-libsigsegv2_2.12-2_amd64.deb ...
Unpacking libsigsegv2:amd64 (2.12-2) ...
Selecting previously unselected package m4.
Preparing to unpack .../02-m4_1.4.18-4_amd64.deb ...
Unpacking m4 (1.4.18-4) ...
Selecting previously unselected package autoconf.
Preparing to unpack .../03-autoconf_2.69-11.1_all.deb ...
Unpacking autoconf (2.69-11.1) ...
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Unpacking autotools-dev (20180224.1) ...
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Preparing to unpack .../05-automake_1%3a1.16.1-4ubuntu6_all.deb ...
Unpacking automake (1:1.16.1-4ubuntu6) ...
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Preparing to unpack .../06-autopoint_0.19.8.1-10build1_all.deb ...
Unpacking autopoint (0.19.8.1-10build1) ...
Selecting previously unselected package libtool.
Preparing to unpack .../07-libtool_2.4.6-14_all.deb ...
Unpacking libtool (2.4.6-14) ...
Selecting previously unselected package dh-autoreconf.
Preparing to unpack .../08-dh-autoreconf_19_all.deb ...
Unpacking dh-autoreconf (19) ...
Selecting previously unselected package libdebhelper-perl.
Preparing to unpack .../09-libdebhelper-perl_12.10ubuntu1_all.deb ...
Unpacking libdebhelper-perl (12.10ubuntu1) ...
Selecting previously unselected package libarchive-zip-perl.
Preparing to unpack .../10-libarchive-zip-perl_1.67-2_all.deb ...
Unpacking libarchive-zip-perl (1.67-2) ...
Selecting previously unselected package libsub-override-perl.
Preparing to unpack .../11-libsub-override-perl_0.09-2_all.deb ...
Unpacking libsub-override-perl (0.09-2) ...
Selecting previously unselected package libfile-stripnondeterminism-perl.
Preparing to unpack .../12-libfile-stripnondeterminism-perl_1.7.0-1_all.deb ...
Unpacking libfile-stripnondeterminism-perl (1.7.0-1) ...
Selecting previously unselected package dh-strip-nondeterminism.
Preparing to unpack .../13-dh-strip-nondeterminism_1.7.0-1_all.deb ...
Unpacking dh-strip-nondeterminism (1.7.0-1) ...
...
Setting up libpcre2-dev:amd64 (10.36-2+ubuntu20.04.1+deb.sury.org+2) ...
Setting up libtool (2.4.6-14) ...
Setting up m4 (1.4.18-4) ...
Setting up intltool-debian (0.35.0+20060710.5) ...
Setting up libmail-sendmail-perl (0.80-1) ...
Setting up autoconf (2.69-11.1) ...
Setting up dh-strip-nondeterminism (1.7.0-1) ...
Setting up automake (1:1.16.1-4ubuntu6) ...
update-alternatives: using /usr/bin/automake-1.16 to provide /usr/bin/automake (automake) in auto mode
Setting up po-debconf (1.0.21) ...
Setting up libltdl-dev:amd64 (2.4.6-14) ...
Setting up php7.3-dev (7.3.27-9+ubuntu20.04.1+deb.sury.org+1) ...
update-alternatives: using /usr/bin/php-config7.3 to provide /usr/bin/php-config (php-config) in auto mode
update-alternatives: using /usr/bin/phpize7.3 to provide /usr/bin/phpize (phpize) in auto mode
Setting up debhelper (12.10ubuntu1) ...
Setting up dh-autoreconf (19) ...
Setting up pkg-php-tools (1.38) ...
Processing triggers for doc-base (0.10.9) ...
Processing 2 added doc-base files...
Processing triggers for libc-bin (2.31-0ubuntu9.2) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for install-info (6.7.0.dfsg.2-5) ...
-EOF-
