PHP Most common errors

This post is related to the common issues we can have when develop in PHP and the way to debug into each one of them:

Nothing is seen. The page is empty and white.
Code doesn't run/what looks like parts of my PHP code are output
Warning: Cannot modify header information - headers already sent
Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given
Warning: [function] expects parameter 1 to be resource, boolean given
Warning: [function]: failed to open stream:
Warning: open_basedir restriction in effect
Warning: Division by zero
Parse error: syntax error, unexpected '['
Parse error: syntax error, unexpected T_XXX
Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE
Parse error: syntax error, unexpected T_VARIABLE
Fatal error: Allowed memory size of XXX bytes exhausted (tried to allocate XXX bytes)
Fatal error: Call to a member function ... on a non-object
Fatal Error: Call to Undefined function XXX
Fatal Error: Cannot redeclare XXX
Fatal error: Can't use function return value in write context
Fatal error: Using $this when not in object context
Notice: Undefined variable
Notice: Undefined Index
Notice: Uninitialized string offset: X
Notice: Use of undefined constant XXX - assumed 'XXX'
Notice: Array to string conversion
MySQL: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near