Keep % sign using sprintf()

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I use %s as placeholders in a text file and something like

$line = sprintf(fgets($fp),$tc,$tc);

To read a line.
So $tc will replace %s in $line.

But if line contains "charge a 2% surcharge" then $tc also replaces the '%'.

How do I get round this and leave '%' untouched and only replace '%s'?
Tried backslash, that doesn't work

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Solved. It is escaped with a double percent like so '%%'