What are common mistakes or pitfalls that can lead to unexpected behavior in PHP code?

One common mistake that can lead to unexpected behavior in PHP code is using loose comparison operators (==) instead of strict comparison operators (===). Loose comparison can result in type coercion, leading to unexpected results. To avoid this, always use strict comparison operators to compare values and types accurately.

// Incorrect usage of loose comparison operator
if($x == 5) {
    // This block will execute even if $x is a string '5'
}

// Correct usage of strict comparison operator
if($x === 5) {
    // This block will only execute if $x is an integer 5
}