Perhaps it would be useful to look at the linux function with the same name.

flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.¹

You asked:

Should using LOCK_SH in read() allow for multiple simultaneous reads?

From the linux manual page for flock:

LOCK_SH

Place a shared lock. More than one process may hold a shared lock for a given file at a given time.

Should the check() method handle the case where the directory does exist but is not writable (i.e. using is_writable())? And what if the user is not able to update permissions?

The flock() function returns true on success or false on failure ² so perhaps it would be useful to use those values to determine if subsequent operations should be attempted.

While using file_put_contents() function were you using LOCK_EX in the $flags (i.e. 3^rd ) parameter)? If not, that might be a technique to simplify the write function.

I can’t think of a reason the write function would need to return the data to be written- is that imperative for your code?

Perhaps it would be useful to look at the manual page for the linux function with the same name:

flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.¹

You asked:

Should using LOCK_SH in read() allow for multiple simultaneous reads?

From the linux manual page for flock:

LOCK_SH

Place a shared lock. More than one process may hold a shared lock for a given file at a given time.

Should the check() method handle the case where the directory does exist but is not writable (i.e. using is_writable())? And what if the user is not able to update permissions?

The flock() function returns true on success or false on failure ² so perhaps it would be useful to use those values to determine if subsequent operations should be attempted.

While using file_put_contents() function were you using LOCK_EX in the $flags (i.e. 3^rd ) parameter)? If not, that might be a technique to simplify the write function.

I can’t think of a reason the write function would need to return the data to be written- is that imperative for your code?