wrong sysntax:

Username='"

If you want a empty value use this

user=''

or

user=""

to escape use [s]bar[/s] backslash before the quote

string='I\'m Batman'

or

string="He say \"I'm not anti-social; I'm just not user friendly\""

right on, thank you for clearing that up

You’re welcome.

In almost all languages (if not all), the quotes have to be the same. What is allowed in between does vary, so it is usually best to look things up yourself. For example, http://dev.mysql.com/doc/refman/5.7/en/string-literals.html or https://www.sqlite.org/lang_expr.html (although the latter seems to be incomplete; “ is also allowed).

The only reason why you should encounter both quotes like you showed is when Username=‘ is part of a string itself. For example:
$inject_str = "' or Username='"; inject_sql($db,$inject_str);

@bolofecal**: “\” is not a “bar” but a backslash; “|” is called a (vertical) bar (or pipe). Also, the backslash escape is not allowed everywhere, including this challenge. Instead, one can use a double single/double quote (e.g.: ‘a’‘b’ or “a”“b”).

dloser,

thanks for the post! that definitely makes it clear to me how the quotations are handled!

Thanks @dloser for correction, sorry about my bad english.

In my country this is bar /
this is inverted bar \