func_num_args() function returns the number of parameters passed in.

100, it's a reference to existing variable.

If you want to include special characters like spaces in the query string, you need to protect them by applying the urlencode() translation function.

Here are three basic types of runtime errors in PHP:
1. Notices: These are trivial, non-critical errors that PHP encounters while executing a script - for example, accessing a variable that has not yet been defined. By default, such errors are not displayed to the user at all - although you can change this default behavior.
2. Warnings: These are more serious errors - for example, attempting to include() a file which does not exist. By default, these errors are displayed to the user, but they do not result in script termination.
3. Fatal errors: These are critical errors - for example, instantiating an object of a non-existent class, or calling a non-existent function. These errors cause the immediate termination of the script, and PHP's default behavior is to display them to the user when they take place.

__sleep returns the array of all the variables than need to be saved, while __wakeup retrieves them.

If you don't want to use the Submit button to submit a form, you can use normal hyper links to submit a form. But you need to use some JavaScript code in the URL of the link. For example:
<a href="javascript: document.myform.submit();">Submit Me</a> 

Since the data inside the single-quoted string is not parsed for variable substitution, it's always a better idea speed-wise to initialize a string with single quotes, unless you specifically need variable substitution.

unlink() is a function for file system handling. It will simply delete the file in context.
unset() is a function for variable management. It will make a variable undefined.

Any time you have an array with more than one dimension, complex parsing syntax is required. print "Contents: {$arr[1][2]}" would've worked.

Yes.