Error while opening PHP file on Plesk-hosted website: PHP Warning: require_once(PEAR/Info.php)

Symptoms

1. Cannot open a PHP file which uses PEAR library:

   CONFIG_TEXT: Internal Server error 500

2. Error in /var/www/vhosts/example.com/logs/error_log

   CONFIG_TEXT: [fcgid:warn] [pid 20442:tid 140600493983488] [client 203.0.113.2:60260] mod_fcgid: stderr: PHP Fatal error: require_once(): Failed opening required 'PEAR/Info.php' (include_path='.:/usr/share/pear:/usr/share/php') in /var/www/vhosts/example.com/httpdocs/pear.php on line 2

Cause

PEAR module is missing from the system. 

Resolution

1. Login to Plesk server over SSH
2. Determine the PEAR package name:

   # pear search info
   WARNING: channel "pear.php.net" has updated its protocols, use "pear channel-update pear.php.net" to update
   Retrieving data...0%
   ..Matched packages, channel pear.php.net:
   =======================================
   Package Stable/(Latest) Local
   File_Infopath 0.1.0 (alpha) A package to read the schema and views in html from a Microsoft Infopath file
   PEAR_Info 1.9.2 (stable) Show Information about your PEAR install and its packages
   PHP_CompatInfo 1.9.0 (stable) Find out the minimum version and the extensions required for a piece of code to run

3. Install the PEAR library:

   # pear install --alldeps PEAR_Info

4. Set the PEAR path for the domain according to instructions from the article.