child.php

<?php

class Child extends Mother
{
    private $fieldsCount;
    
    public function  __construct()
    {
        $this->fieldsCount = 5;
    }

    protected function prepareColumns()
    {
        // ...
    }
}

mother.php

<?php

abstract class Mother
{
    abstract protected function prepareColumns();

    public function printData()
    {
        $this->prepareColumns();
    }
}