I reckon Alex has it right. And any assembler/machine code programmer will tell you that is the escape key. If you can’t search for ascii 27 (0x1B) try this: -
Get notepad++ Downloads | Notepad++
Open all your .php’s in it.
Edit menu, character panel.
Go to 27 and double click the word ESC in the Character column.
That will insert that character into the current document at the cursor.
Now cut that to the clipboard.
Ctrl + F for find.
Paste your ESC character, it’ll look like a square now
Click the “Find All in All Opened Documents”
And use the search results panel to hunt it down.
Remove and save!
Edit: Check Z’s post after mine for the correct solution - but if ever you can’t find a random control character (ascii value < 32) then use my approach.