If you want to debug a large amount of data. You need to create a Log file with the following piece of code.

$file = fopen ( "testfile.txt" , "a+" );
fwrite ( $file , serialize ( print_r ( $_POST , true ) ) );

fopen(): It is inbuilt function which opens the file & if it does not able to find file then it will automatically create a new file with name given as a parameter.

a+ defines that if you want to write data continues in your file.

fwrite() : It is php inbuilt function which writes the content in the file that you specified.

$file: It is variable name that assign to the created file.

serialize() : This functions serialize the data which comes in variable $output.

print_r(): It is a php inbuilt function used to print the array.