Please read the doc carefully... The available attributes filter are :
+--------------------+----------------------------------------------------------------------------------------+
| Filter | Description |
+--------------------+----------------------------------------------------------------------------------------+
| [attribute] | Matches elements that have the specified attribute. |
| [!attribute] | Matches elements that don't have the specified attribute. |
| [attribute=value] | Matches elements that have the specified attribute with a certain value. |
| [attribute!=value] | Matches elements that don't have the specified attribute with a certain value. |
| [attribute^=value] | Matches elements that have the specified attribute and it starts with a certain value. |
| [attribute$=value] | Matches elements that have the specified attribute and it ends with a certain value. |
| [attribute*=value] | Matches elements that have the specified attribute and it contains a certain value. |
+--------------------+----------------------------------------------------------------------------------------+
So in your case, you can use the last one for example [the following is tested and it works]:
$lis = $dom->find('[style*="font-family:symbol"]');