Hi I assume you mean:
Select * from MyTable Where Field Like "%ABC/DEF%"
rather than ...
"Select * from MyTable Where Field Like "%ABC/DEF%"
... since the the second won't work in phpMyAdmin or in a script.
This should work:
mysql_query('Select * from MyTable Where Field Like "%ABC/DEF%"');
If not, show us the exact PHP snippet of code you are using to run the query (including connection code - minus any sensitive info like passwords of course)
Paul