How to connect PHP 5.6 to SQL db after compilation

I installed PHP 5.6 from source on Debian server 9 server. The OpenSSL version in Debian 9 is too new for PHP 5.6, so I had to compile an older version in /opt/openssl to use it with PHP 5.6. ( version openssl-1.0.1t )

I configured the compilation of PHP5.6 like this :

./configure --prefix=/opt/PHP/php-5.6 --with-pdo-pgsql --with-zlib-dir --with-freetype-dir --enable-mbstring --with-libxml-dir=/usr --enable-soap --enable-calendar --with-curl --with-mcrypt --with-zlib --with-pgsql --disable-rpath --enable-inline-optimization --with-bz2 --with-zlib --enable-sockets --enable-sysvsem --enable-sysvshm --enable-pcntl --enable-mbregex --enable-exif --enable-bcmath --with-mhash --enable-zip --with-pcre-regex --with-pdo-mysql --with-mysqli --with-mysql-sock=/var/run/mysqld/mysqld.sock --with-jpeg-dir=/usr --with-png-dir=/usr --enable-gd-native-ttf --with-openssl=/opt/openssl --with-fpm-user=www-data --with-fpm-group=www-data --with-libdir=/lib/x86_64-linux-gnu --enable-ftp --with-kerberos --with-gettext --with-xmlrpc --with-xsl --enable-opcache --enable-fpm

I've got PHP5.6 working after the command make and make install

I writed this PHP script for testing the connection ( I hided the good value as well ) :

    <?php
$servername = "localhost";
$username = "username";
$password = "password";

try {
    $conn = new PDO("mysql:host=$servername;dbname=myDB", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected successfully"; 
    }
catch(PDOException $e)
    {
    echo "Connection failed: " . $e->getMessage();
    }
?>

I tried it like this and I got :

 /opt//PHP/php-5.6/bin/php connect_db.php
Connection failed: SQLSTATE[HY000] [2002] No such file or directory

On the same server, I compiled PHP7.1 from source with this configuration of compilation :

./configure --prefix=/opt/PHP/php-7.1 --with-pdo-pgsql --with-zlib-dir --with-freetype-dir --enable-mbstring --with-libxml-dir=/usr --enable-soap --enable-calendar --with-curl --with-zlib --with-gd --with-pgsql --disable-rpath --enable-inline-optimization --with-bz2 --with-zlib --enable-sockets --enable-sysvsem --enable-sysvshm --enable-pcntl --enable-mbregex --enable-exif --enable-bcmath --with-mhash --enable-zip --with-pcre-regex --with-pdo-mysql --with-mysqli --with-mysql-sock=/var/run/mysqld/mysqld.sock --with-jpeg-dir=/usr --with-png-dir=/usr --with-openssl --with-fpm-user=www-data --with-fpm-group=www-data --with-libdir=/lib/x86_64-linux-gnu --enable-ftp --with-imap --with-imap-ssl --with-kerberos --with-gettext --with-xmlrpc --with-xsl --enable-opcache --enable-fpm

I tried the same PHP script and it passed well :

/opt//PHP/php-7.1/bin/php connect_db.php
Connected successfully

How to configure the compilation of PHP5.6 for get the same with PHP7.1 component ?