MySQL is a service, it allows multiple connections at once, is easy to manage,... Seriously, if you learn how to use MySQL in PHP, you won't
EVER want to use *.txt files anymore!
Imagine two almost simultaneous attempts to write to a single text file to update some stuff. The first instance reads it, then the second does. The first one writes some stuff to it, but the second one overwrites it without knowing that the first instance just wrote something else to that file. Thus, your first writing action will be lost. On a small server, this may not sound like a big problem, but imagine having a popular site with lots of visitors... then you'll be happy you've chosen for MySQL!
Don't know much about SQLite, but if Wikipedia is correct, it uses a .sqlite file to store the information in... Uh?
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