core-bundle/src/Resources/contao/library/Contao/Model.php
public function markModified($strKey)
	{
		if (!isset($this->arrModified[$strKey]))
		{
			$this->arrModified[$strKey] = $this->arrData[$strKey] ?? null;
		}
	}
public function markModified($strKey)
	{
		if (!isset($this->arrModified[$strKey]))
		{
			$this->arrModified[$strKey] = $this->arrData[$strKey] ?? null;
		}
	}
Mark a field as modified

@param string $strKey The field key