Warnings, Notices and Deprecated Messages

Warnings are the messages you will see most while learning. Each has a standard cause and a standard fix.

Undefined variable

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<?php
echo $total;

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Warning: Undefined variable $total in /path/to/index.php on line 2

Cause: you used a variable before giving it a value, or you mistyped its name. Fix: set the variable first, check the spelling, or use $total ?? 0.

Undefined array key

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<?php
$user = ["name" => "Ana"];

echo $user["email"];

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Warning: Undefined array key "email" in /path/to/index.php on line 4

Cause: the key does not exist. Fix: $user["email"] ?? "", or check with isset() or array_key_exists().

Array to string conversion

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<?php
$items = ["a", "b"];

echo "Items: " . $items;

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Warning: Array to string conversion in /path/to/index.php on line 4
Items: Array

Cause: you printed an array as if it were text. Fix: implode(", ", $items) or loop through it.

Trying to read a property of null

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<?php
$user = null;

echo $user->name;

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Warning: Attempt to read property "name" on null in /path/to/index.php on line 4

Cause: a value you expected to be an object is null (for example, nothing was found). Fix: check for null, or use the nullsafe operator: $user?->name.

A non-numeric value

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<?php
echo "5 apples" + 3;

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Warning: A non-numeric value encountered in /path/to/index.php on line 2
8

Fix: validate input before using it in maths.

Headers already sent

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<?php
echo "Hello";
header("Location: /home");

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Hello
Warning: Cannot modify header information - headers already sent by (output started at /path/to/index.php:2) in /path/to/index.php on line 3

Cause: header(), setcookie() or session_start() was called after output. Fix: move them before any output. Look for stray spaces before <?php too.

Deprecated

A message like Deprecated: ... is deprecated means a function or a way of writing will disappear. Examples are passing null to a function that expects a string (deprecated in PHP 8.1), or using ${var} inside strings (PHP 8.2).

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<?php
echo strlen(null);

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Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in /path/to/index.php on line 2
0

Fix: pass a real string, or handle the null first.

How to deal with them

1. Read the message and the line number.
2. Look at that line and the one before it.
3. Fix the cause, do not hide the message.

The @ operator hides errors: avoid it

@ in front of an expression silences its warnings. It hides the problem and makes bugs hard to find.

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<?php
$value = @$undefined["x"];   // hides the warning, the bug remains
var_dump($value);

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NULL

Use ?? or a proper check instead.