When ezmysqli is configured eZ correctly uses ezmysqlidb.php, however in ezpublish_legacy/lib/ezdb/classes/ezmysqlidb.php within function escapeString the MySQL-function mysql_escape_string is used.

mysql_escape_string is deprecated in PHP 4.3 and removed in PHP 7, so eZ Publish won't work with PHP 7.

ezpublish_legacy/lib/ezdb/classes/ezmysqlidb.php:
eZDebug::writeDebug( 'escapeString called before connection is made', _METHOD_ );
return mysql_escape_string( $str );