OK, so unlink() won't wait, and that's fine.
But what happens to the script(a) what opens and locks
a file? when unlink() was done by another script(b),
can the script(a) get the content of the file?

Let's see...

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(1) Open two terminals.

(2) Execute "php lock-and-sleep.php" in a terminal and
	Execute "php unlink-it.php" in another terminal

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if a file was deleted by another process,
file_get_contents emits waning:
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PHP Warning:  file_get_contents(/var/www/phpkobo/research/php-basic/file-lock/unlink-locked-file2/data/test.txt): failed to open stream: No such file or directory in /var/www/phpkobo/research/php-basic/file-lock/unlink-locked-file2/lock-and-sleep.php on line 31
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raises and return false
because it can't find it.
simply put "@" in front of "file_get_contents"
to surpress the warning.