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Compilation of Errors and Warnings

Notice: Undefined index

Section titled “Notice: Undefined index”

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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

Section titled “Warning: Cannot modify header information - headers already sent”

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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
    # 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

    Section titled “Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM”

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    “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.