oop - Reference - What does this error mean in PHP?

What is
this?

This is a number of answers about
warnings, errors, and notices you might encounter while programming PHP and have no clue
how to fix them. This is also a Community Wiki, so everyone is invited to participate
adding to and maintaining this list.

Why is
this?

Questions like href="https://stackoverflow.com/a/12770075">"Headers already sent" or href="https://stackoverflow.com/a/12769983">"Calling a member of a
non-object" pop up frequently on . The root cause of those questions is always
the same. So the answers to those questions typically repeat them and then show the OP
which line to change in their particular case. These answers do not add any value to the
site because they only apply to the OP's particular code. Other users having the same
error cannot easily read the solution out of it because they are too localized. That is
sad because once you understood the root cause, fixing the error is trivial. Hence, this
list tries to explain the solution in a general way to
apply.

What should I do
here?

If your question has been marked as a
duplicate of this one, please find your error message below and apply the fix to your
code. The answers usually contain further links to investigate in case it shouldn't be
clear from the general answer alone.

If you want
to contribute, please add your "favorite" error message, warning or notice, one per
answer, a short description what it means (even if it is only highlighting terms to
their manual page), a possible solution or debugging approach and a listing of existing
Q&A that are of value. Also, feel free to improve any existing
answers.

The
List

• href="https://stackoverflow.com/a/12772851">Nothing is seen. The page is empty and
  white. (also known as White Page/Screen Of
  Death)



• href="https://stackoverflow.com/a/24423272">Code doesn't run/what looks like parts of
  my PHP code are output


• href="https://stackoverflow.com/a/12770075">Warning: Cannot modify header information
  - headers already sent


• href="https://stackoverflow.com/a/12770072">Warning: mysql_fetch_array() expects
  parameter 1 to be resource, boolean given a.k.a.
  href="https://stackoverflow.com/a/12770072">Warning: mysql_fetch_array(): supplied
  argument is not a valid MySQL result resource a.k.a.
  href="https://stackoverflow.com/a/12770072">Warning: mysqli_num_rows() expects
  parameter 1 to be mysqli_result, boolean given (or similar
  variations)


• href="https://stackoverflow.com/a/14067623">Warning: [function]
  expects parameter 1 to be resource, boolean
  given


• href="https://stackoverflow.com/a/12820243">Warning: [function]:
  failed to open stream:
  [reason]


• href="https://stackoverflow.com/a/12772244">Warning: open_basedir restriction in
  effect


• href="https://stackoverflow.com/a/21507314">Warning: Division by
  zero


• href="https://stackoverflow.com/a/27678436">Warning: Illegal string offset
  'XXX'


• href="https://stackoverflow.com/a/56011600">Warning: count(): Parameter must be an
  array or an object that implements
  Countable


• href="https://stackoverflow.com/a/22316776">Parse error: syntax error, unexpected
  '['



• href="https://stackoverflow.com/a/12770089">Parse error: syntax error, unexpected
  T_XXX


• href="https://stackoverflow.com/a/13935532">Parse error: syntax error, unexpected
  T_ENCAPSED_AND_WHITESPACE


• href="https://stackoverflow.com/a/12770129">Parse error: syntax error, unexpected
  T_PAAMAYIM_NEKUDOTAYIM


• href="https://stackoverflow.com/q/23726288">Parse error: syntax error, unexpected
  'require_once' (T_REQUIRE_ONCE), expecting function
  (T_FUNCTION)


• href="https://stackoverflow.com/a/15539535">Parse error: syntax error, unexpected
  T_VARIABLE


• href="https://stackoverflow.com/a/12770179">Fatal error: Allowed memory size of XXX
  bytes exhausted (tried to allocate XXX
  bytes)


• href="https://stackoverflow.com/a/12769983">Fatal error: Call to a member function
  ... on a non-object or null


• href="https://stackoverflow.com/a/12784048">Fatal Error: Call to Undefined function
  XXX


• href="https://stackoverflow.com/a/13879461">Fatal Error: Cannot redeclare
  XXX


• href="https://stackoverflow.com/a/12770530">Fatal error: Can't use function return
  value in write context



• href="https://stackoverflow.com/q/1885302">Fatal error: Declaration of AAA::BBB()
  must be compatible with that of
  CCC::BBB()'


• href="https://stackoverflow.com/a/12770317">Fatal error: Using $this when not in
  object context


• href="https://stackoverflow.com/a/56658110">Fatal error: Object of class Closure
  could not be converted to string


• href="https://stackoverflow.com/a/59619392/1255289">Fatal error: Undefined class
  constant


• href="https://stackoverflow.com/a/24507107">Notice: Array to string
  conversion


• href="https://stackoverflow.com/a/26572398">Notice: Trying to get property of
  non-object error


• href="https://stackoverflow.com/a/12778634">Notice: Undefined variable or
  property


• href="https://stackoverflow.com/a/12770836">Notice: Undefined
  Index


• href="https://stackoverflow.com/a/21793407">Notice: Undefined offset XXX
  [Reference]


• href="https://stackoverflow.com/a/17630869">Notice: Uninitialized string offset:
  XXX



• href="https://stackoverflow.com/a/12773272">Notice: Use of undefined constant XXX -
  assumed 'XXX'


• href="https://stackoverflow.com/a/12771239">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 ... at line ...


• href="https://stackoverflow.com/a/21942832">Strict Standards: Non-static method
  [::] should not be called
  statically


• href="https://stackoverflow.com/a/52771226">Warning: function expects parameter X to
  be boolean/string/integer


• href="https://stackoverflow.com/a/53843354">HTTP Error 500 - Internal server
  error


• href="https://stackoverflow.com/a/59158847">Deprecated: Array and string offset
  access syntax with curly braces is
  deprecated

Also,
see: