As always

yum --enablerepo=remi install php-igbinary

Activation, for the sessions (will save some space), in the /etc/php.d/igbinary.ini file:

session.serialize_handler=igbinary

Use in some code :

$a=igbinary_serialize(123);
$b=igbinary_unserialize($a); 
echo "A:".hexdump($a)."\nB:$b\n";

New RPM of php-pecl-memcached can also use this serializer.