Assignment vs. Comparison

One of the most common errors in PHP is using the assignment operator (=) instead of the comparison operator (== or ===) in conditions:

// WRONG: This always evaluates to true (unless $value is falsy)
if ($value = 10) {
    echo "This will execute!";
}

// CORRECT: This evaluates to true only if $value equals 10
if ($value == 10) {
    echo "This will execute only if $value is 10.";
}
                

The first example assigns 10 to $value and then checks if that result is truthy (which it is), rather than comparing $value to 10.

Missing Braces for Multi-line Blocks

If you omit braces, only the first statement is controlled by the if or else:

// WRONG: Only the first echo is part of the if branch
if ($isLoggedIn)
    echo "Welcome back!";
    echo "You have new messages."; // This always executes!

// CORRECT: Both echoes are part of the if branch
if ($isLoggedIn) {
    echo "Welcome back!";
    echo "You have new messages.";
}
                

Forgetting that 0 and Empty Strings are Falsy

Be careful when checking numeric or string variables that might legitimately be 0 or empty:

$count = 0;

// WRONG: This condition fails even though $count is set
if ($count) {
    echo "Items found: $count";
} else {
    echo "No items found."; // This will execute!
}

// CORRECT: Explicitly check for null or use the !== operator
if ($count !== null) {
    echo "Items found: $count"; // This will execute with 0
} else {
    echo "No items found.";
}
                

Not Using Strict Comparison When Needed

Remember the difference between loose (==) and strict (===) comparison:

$userInput = "42";

// WRONG: This will evaluate to true due to type juggling
if ($userInput == 42) {
    echo "Equal!"; // This will execute!
}

// CORRECT: This will evaluate to false because types differ
if ($userInput === 42) {
    echo "Equal!"; // This won't execute
} else {
    echo "Not equal! String vs Integer"; // This will execute
}