%s is a C format specifier for a string.

msg = g_strdup_printf (_("<b><big>Enter your password to run "
                         "the application '%s' as user %s"
                         "</big></b>"),
                         command, context->user);

means "where you see the first %s, replace it with the contents of command as a string, and where you see the second %s, replace it with the contents of context->user as a string.

This is really more of a general PHP programming question and might get closed for that reason. But before that happens, let's see if I can't clear some of this up for you.

Those strings that start with % signs are in "printf format". For example, %2$s translates to "replace this token with the second parameter, and treat it like a string".

When you see a variable in curly braces inside a string, like "Today is {$day}", that's the same as putting the variable there without curly braces, like "Today is $day". But formatting the variable like this makes it stand out, so it's more clear there's a variable there. It also prevents other characters next to the variable name from getting interpreted as part of the name. Without curly braces, would PHP know what to do with "Your robot name would be {$firstName}Number5"? It would see "$firstNameNumber5" and look for a variable named $firstNameNumber5 instead of simply $firstName.

I'm not sure what you're asking about in the last example. Is it the __FILE__ you're confused by? That's a PHP Magic Constant that gets replaced with the full path to the file that bit of code is in.

They are used for formatting strings. %s acts a placeholder for a string while %d acts as a placeholder for a number. Their associated values are passed in via a tuple using the % operator.

name = 'marcog'
number = 42
print '%s %d' % (name, number)

will print marcog 42. Note that name is a string (%s) and number is an integer (%d for decimal).

See https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting for details.

In Python 3 the example would be:

print('%s %d' % (name, number))

It tells print to output a string (%s) and a signed integer (%d). This is more useful if you want to have a more complicated output. For example:

print("Hello %s your number is %d" % (name, number))

With the sample values you've given, this will output:

Hello marcog your number is 42

This is undefined behavior, anything can happen included something that looks like correct. But it is incorrect. Your compiler can probably tell you the problem if you use correct options.

Standard says (emphasized is mine):

7.21.6.1 The fprintf function

2. The fprintf function writes output to the stream pointed to by stream, under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered.