You can bold part of the text in a cell using rich text formatting, as described in section 4.6.37 of the developer documentation.
$objRichText = new PHPExcel_RichText();
$objRichText->createText('This text is ');
$objBold = $objRichText->createTextRun('bold');
$objBold->getFont()->setBold(true);
$objRichText->createText(' within the cell.');
$objPHPExcel->getActiveSheet()->getCell('A18')->setValue($objRichText);
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