# pecl install idn
Failed to download pecl/idn within preferred state "stable", latest release is version 0.2.0, stability "beta", use "channel://pecl.php.net/idn-0.2.0" to install
install failed
# pecl install channel://pecl.php.net/idn-0.2.0
downloading idn-0.2.0.tgz ...
Starting to download idn-0.2.0.tgz (6,313 bytes)
.....done: 6,313 bytes
3 source files, building
running: phpize
Configuring for:
PHP Api Version:         20100412
Zend Module Api No:      20100525
Zend Extension Api No:   220100525
GNU libidn installation dir? [autodetect] :
building in /var/tmp/pear-build-rootw5Z2Gw/idn-0.2.0
running: /var/tmp/idn/configure --with-php-config=/usr/bin/php-config --with-idn
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking for a sed that does not truncate output... /usr/bin/sed
checking for cc... cc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether cc accepts -g... yes
checking for cc option to accept ISO C89... none needed
checking how to run the C preprocessor... cc -E
checking for icc... no
checking for suncc... no
checking whether cc understands -c and -o together... yes
checking for system library directory... lib
checking if compiler supports -R... no
checking if compiler supports -Wl,-rpath,... yes
checking build system type... x86_64-unknown-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking target system type... x86_64-unknown-linux-gnu
checking for PHP prefix... /usr
checking for PHP includes... -I/usr/include/php -I/usr/include/php/main -I/usr/include/php/TSRM -I/usr/include/php/Zend -I/usr/include/php/ext -I/usr/include/php/ext/date/lib
checking for PHP extension directory... /usr/lib64/php/modules
checking for PHP installed headers prefix... /usr/include/php
checking if debug is enabled... no
checking if zts is enabled... no
checking for re2c... re2c
checking for re2c version... 0.14.3 (ok)
checking for gawk... gawk
checking for GNU Libidn support... yes, shared
checking for idn files in default path... not found
configure: error: Please reinstall the GNU Libidn distribution
ERROR: `/var/tmp/idn/configure --with-php-config=/usr/bin/php-config --with-idn' failed
# rpm -q libidn
libidn-1.28-4.el7.x86_64

NOTE:
# rpm -ql libidn | grep -v share
/usr/bin/idn
/usr/lib64/libidn.so.11
/usr/lib64/libidn.so.11.6.11

I tried using all these in place of "autodetect". Same failure for each.