Compilation of Errors and Warnings

Notice: Undefined index

Appearance :

Trying to access an array by a key that does not exist in the array

Possible Solution :

Check the availability before accessing it. Use:

  1. isset()
  2. array_key_exists()

Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM

Appearance:

"Paamayim Nekudotayim" means "double colon" in Hebrew; thus this error refers to the inappropriate use of the double colon operator (::). The error is typically caused by an attempt to call a static method that is, in fact, not static.

Possible Solution:

$classname::doMethod();

If the above code causes this error, you most likely need to simply change the way you call the method:

$classname->doMethod();

The latter example assumes that $classname is an instance of a class, and the doMethod() is not a static method of that class.

Warning: Cannot modify header information - headers already sent

Appearance :

Happens when your script tries to send a HTTP header to the client but there already was output before, which resulted in headers to be already sent to the client.

Possible Causes :

  1. Print, echo: Output from print and echo statements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that.

  2. Raw HTML areas: Unparsed HTML sections in a .php file are direct output as well. Script conditions that will trigger a header() call must be noted before any raw blocks.

    <!DOCTYPE html>
    <?php
         // Too late for headers already.
    
  3. Whitespace before <?php for "script.php line 1" warnings: If the warning refers to output in line 1, then it's mostly leading whitespace, text or HTML before the opening <?php token.

    <?php
    # There's a SINGLE space/newline before <? - Which already seals it.
    

Reference from SO answer by Mario