# 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()

# 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 :

  • **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.
  • **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.
    
    
  • **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

    # 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.