PHP Version 5.6.38

        Configuration File (php.ini) Path   /usr/local/apps/php56/etc
        Loaded Configuration File   /usr/local/apps/php56/etc/php.ini
        Scan this dir for additional .ini files     /usr/local/apps/php56/etc/php.d
        Additional .ini files parsed    /usr/local/apps/php56/etc/php.d/extra.ini

        extension_dir   /usr/local/apps/php56/ext/

yum install php-pgsql

/usr/lib64/php/modules/pgsql.so
/usr/lib64/php/modules/pdo_pgsql.so

# cat /usr/local/apps/php56/etc/php.d/extra.ini
extension=bcmath.so
extension=bz2.so
extension=calendar.so
extension=ctype.so
extension=curl.so
extension=dba.so
extension=exif.so
extension=ftp.so
;extension=gd.so
extension=gettext.so
extension=gmp.so
extension=iconv.so
extension=imap.so
extension=intl.so
extension=ldap.so
extension=mbstring.so
extension=mcrypt.so
extension=mysql.so
extension=mysqli.so
extension=mysqlnd.so
extension=openssl.so
extension=pcntl.so
extension=pdo.so
extension=pdo_mysql.so
extension=pdo_pgsql.so
extension=pdo_sqlite.so
extension=pgsql.so
extension=shmop.so
extension=soap.so
extension=sockets.so
extension=sqlite3.so
extension=sysvsem.so
extension=sysvshm.so
extension=tidy.so
extension=tokenizer.so
extension=wddx.so
extension=xmlrpc.so
extension=xsl.so
extension=zip.so
extension=zlib.so
extension=imagick.so
;extension=libjpeg.so
;extension=libjpeg.so.9

print_r(get_loaded_extensions());

Array ( [0] => Core [1] => date [2] => ereg [3] => libxml [4] => pcre [5] => dom [6] => fileinfo [7] => filter [8] => hash [9] => json [10] => SPL [11] => bz2 [12] => posix [13] => Reflection [14] => session [15] => SimpleXML [16] => standard [17] => xml [18] => xmlreader [19] => xmlwriter [20] => cgi-fcgi [21] => bcmath [22] => openssl [23] => calendar [24] => ctype [25] => curl [26] => dba [27] => mbstring [28] => ftp [29] => gettext [30] => gmp [31] => iconv [32] => imap [33] => intl [34] => ldap [35] => exif [36] => mcrypt [37] => mysqlnd [38] => mysqli [39] => mysql [40] => zlib [41] => pcntl [42] => PDO [43] => pdo_mysql [44] => pdo_sqlite [45] => shmop [46] => soap [47] => sockets [48] => sqlite3 [49] => sysvsem [50] => sysvshm [51] => tidy [52] => tokenizer [53] => wddx [54] => xmlrpc [55] => xsl [56] => zip [57] => Phar [58] => imagick [59] => mhash [60] => ionCube Loader [61] => Zend OPcache )