I have been learning the PHPWord for my academic project, and I also have the latest PHPWord library which supports "vMerge" for rowspan and "gridSpan" f
There is an example in the samples that is quite close to what you are doing: https://github.com/PHPOffice/PHPWord/blob/develop/samples/Sample_09_Tables.php
and modifying that a bit to achieve your example:
$cellRowSpan = array('vMerge' => 'restart');
$cellRowContinue = array('vMerge' => 'continue');
$cellColSpan = array('gridSpan' => 2);
$table->addRow();
$table->addCell(2000, $cellRowSpan)->addText("1");
$table->addCell(2000, $cellRowSpan)->addText("2");
$table->addCell(4000, $cellColSpan)->addText("3");
$table->addCell(2000, $cellRowSpan)->addText("6");
$table->addRow();
$table->addCell(null, $cellRowContinue);
$table->addCell(null, $cellRowContinue);
$table->addCell(2000)->addText("4");
$table->addCell(2000)->addText("5");
$table->addCell(null, $cellRowContinue);
$table->addRow();
$table->addCell(2000);
$table->addCell(2000);
$table->addCell(2000);
$table->addCell(2000);
$table->addCell(2000);
tested with 0.13.0 version (for some reason libreoffice didn't like the two adjecent cells with vMerge continue definitions and didn't display them as expected, but word did display them nicely as expected)