"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP

I'm running a PHP script and continue to receive errors like: > Notice: Undefined variable: my_variable_name in C:\wamp\www\mypath\index.php on line 10 > > Notice: Undefined index: my_index C:\wamp\www\mypath\index.php on line 11 > > Warning: Undefined array key "my_index" in C:\wamp\www\mypath\index.php on line 11 Line 10 and 11 looks like this: echo "My variable value is: " . $my_variable_name; echo "My index value is: " . $my_array["my_index"]; What is the meaning of these error messages? Why do they appear all of a sudden? I used to use this script for years and I've never had any problem. How do I fix them? <hr/> > <sub>**This is a General Reference question** for people to link to as duplicate, instead of having to explain the issue over and over again. I feel this is necessary because most real-world answers on this issue are very specific. </sub> > > <sub>Related Meta discussion:</sub> > > * <sub>[What can be done about repetitive questions?][1]</sub> > * <sub>[Do “reference questions” make sense?][2]</sub> [1]: https://meta.stackexchange.com/questions/62258/what-can-be-done-about-repetitive-questions [2]: https://meta.stackexchange.com/questions/63762/do-reference-questions-make-sense