I don't understan why grep doesn't work in the first example
bla@ble:~/html/example$ grep -r "protected $disallowedBlockNames = array('install/end');" app/
bla@ble:~/html/example$
But
bla@ble:~/html/example$ grep -r 'protected $disallowedBlockNames = array' app/
app/Resource/Block.php: protected $disallowedBlockNames = array('install/end');
You didn't provide sample input but in your first example your double quotes are allowing the disallowedBlockNames variable to be expanded by your shell before it is used by grep. I'm assuming this is a variable set in your php code and does not exist in your shell and therefore it is expanding to nothing. So what you are really sending to grep is:
grep -r "protected = array('install/end');" app/
In the second example the single quotes prevent the shell from expanding the variable.
$ you must escape it either with single quotes or with the \ escape character.