PHP remains one of the most popular languages for web development due to its ease of use. But the more it evolves, the more nuances it has that can stump developers and complicate debugging. Below are ten of the most common mistakes that even experienced PHP programmers make.

Mistake #1. “Hanging” references after a foreach loop

When using foreach with references, you can accidentally leave a variable that continues to refer to the last element of the array even after the loop has finished. This leads to unexpected data changes in subsequent sections of the code.

How to avoid it: delete the reference with unset() immediately after the loop ends.

Mistake #2. Misunderstanding the behavior of isset()

isset() returns false not only when the variable is missing, but also when it is null. This easily causes logical errors: checking for the existence of a key and checking its value are not the same thing.

How to avoid it:

  • Use array_key_exists() to check for the existence of a key in an array.
  • To check for the existence of a variable in the current scope, you can use a combination with get_defined_vars().

Mistake #3. Confusion between “by value” and “by reference” return

Arrays and variables in PHP are passed by value, while objects are passed by reference. Because of this, developers are often surprised that changes are not saved or that they are referring to different copies of data.

How to avoid it:

  • Clearly understand what the method returns: a copy, a reference, or an object.
  • Avoid returning private data via a reference—this violates encapsulation. It is better to use getters and setters.

Mistake #4. SQL queries in a loop

Accessing the database within a loop leads to a huge number of queries. For example, when processing an array of thousands of elements, a thousand queries are executed — this slows down the work and can even crash the system.

How to avoid it:

  • Form a single general query that gets all the necessary data at once.
  • Use a list selection (via IN or other mechanisms).

Mistake #5. Irrational use of memory

When selecting too large amounts of data, especially in combination with an outdated libmysqlclient library, PHP can unexpectedly consume huge amounts of memory. Often, monitoring shows less memory usage than there actually is, which is misleading.

How to avoid it:

  • Use the mysqlnd driver, which works with memory more correctly.
  • Divide the query into portions instead of loading huge arrays in their entirety.

Mistake #6. Ignoring Unicode and UTF-8

Despite the popularity of UTF-8, many developers continue to write code as if all strings were ASCII. This results in problems with string length, JSON, databases, and hardcoding.

How to avoid it:

  • Use the mbstring extension and mb_* functions.
  • Configure databases to work with UTF-8.
  • Control the encoding of project files.
  • Remember that different functions (e.g., serialization and JSON) handle Unicode differently.

Mistake #7. Expecting $_POST to always be populated

If data is sent in JSON format, PHP does not fill $_POST by itself. Developers often wonder why the array is empty even though the request has been sent.

How to avoid it:

  • If POST data comes in JSON format, you need to read the request body yourself and convert it to an array.

Mistake #8. Assuming that PHP supports the char type

PHP does not have a separate character type — only strings. String increment behaves differently than developers expect. For example, increasing a letter can generate two-character strings, which breaks the logic of loops and comparisons.

How to avoid it:

  • Use numeric representations of characters or pre-prepared arrays of letters.

Mistake #9. Ignoring coding standards

The lack of uniform rules for writing code leads to chaos: the code becomes difficult to read, difficult to maintain, and easily broken.

How to avoid it:

  • Use PSR standards: PSR-1, PSR-2, PSR-4, and others.
  • Follow the style accepted by your team or industry.

Mistake #10. Incorrect use of empty()

empty() behaves unpredictably with objects, especially if the class uses the magic method __get(). Sometimes empty() considers a value to be empty even when it is not.

How to avoid it:

  • Use count() to check arrays.
  • Be careful when using empty() with objects.
  • Keep in mind that magic methods can change expected behavior.

Conclusion

PHP seems simple, but beneath the surface there are many subtleties that can lead to unexpected errors. Understanding the peculiarities of the language’s behavior, paying close attention to working with strings, memory, database queries, and superglobal arrays allows you to write reliable, scalable, and predictable code.

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