SUMMARY
The PHP Runtime Configuration Docs give you some idea why:
Enabling E_NOTICE during development has some benefits.
For debugging purposes: NOTICE messages will warn you about possible bugs in your code. For example, use of unassigned values is warned. It is extremely useful to find typos and to save time for debugging.
NOTICE messages will warn you about bad style. For example, $arr[item] is better to be written as $arr[‘item’] since PHP tries to treat “item” as constant. If it is not a constant, PHP assumes it is a string index for the array.
Here’s a more detailed explanation of each…
1. TO DETECT TYPOS
The main cause of E_NOTICE errors is typos.
Example – notice.php
<?php
$username="joe"; // in real life this would be from $_SESSION
// and then much further down in the code...
if ($usernmae) { // typo, $usernmae expands to null
echo "Logged in";
}
else {
echo "Please log in...";
}
?>
Output without E_NOTICE
Please log in...
Wrong! You didn’t mean that!
Output with E_NOTICE
Notice: Undefined variable: usernmae in /home/user/notice.php on line 3
Please log in...
In PHP, a variable that doesn’t exist will return null rather than causing an error, and that could cause code to behave differently than expected, so it’s best to heed E_NOTICE warnings.
2. TO DETECT AMBIGUOUS ARRAY INDEXES
It also warns you about array indexes that might change on you, e.g.
Example – code looks like this today
<?php
$arr = array();
$arr['username'] = 'fred';
// then further down
echo $arr[username];
?>
Output without E_NOTICE
fred
Example – tomorrow you include a library
<?php
// tomorrow someone adds this
include_once('somelib.php');
$arr = array();
$arr['username'] = 'fred';
// then further down
echo $arr[username];
?>
and the library does something like this:
<?php
define("username", "Mary");
?>
New output
Empty, because now it expands to:
echo $arr["Mary"];
and there is no key Mary in $arr.
Output with E_NOTICE
If only the programmer had E_NOTICE on, PHP would have printed an error message:
Notice: Use of undefined constant username - assumed 'username' in /home/user/example2.php on line 8
fred
3. THE BEST REASON
If you don’t fix all the E_NOTICE errors that you think aren’t errors, you will probably grow complacent, and start ignoring the messages, and then one day when a real error happens, you won’t notice it.