Loop Mistakes to Avoid

Loops are simple, but a few mistakes show up again and again. Learn to spot them early.

1. The infinite loop

A loop that never reaches its stop condition runs until PHP’s max_execution_time ends the script.

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<?php
$i = 1;
while ($i <= 5) {
    echo $i;
    // forgot $i++ so the condition is always true
}

PHP stops a web request after the time limit (30 seconds by default). On the command line, there is no limit, so press Ctrl+C. Before running a new loop, say out loud what makes it stop.

2. Off-by-one

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

for ($i = 0; $i <= count($items); $i++) {
    echo $items[$i] ?? "(nothing)";
}

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abc(nothing)

The loop ran one time too many. <= count() should be < count(). (Without ??, you would see a “Undefined array key 3” warning.)

3. Changing an array while you loop through it

Adding or removing items of the array you are looping over leads to confusing results. Build a new array instead.

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<?php
$numbers = [1, 2, 3, 4, 5, 6];
$odd = [];

foreach ($numbers as $n) {
    if ($n % 2 !== 0) {
        $odd[] = $n;
    }
}

echo implode(", ", $odd);

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1, 3, 5

4. Forgetting to unset a reference

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<?php
$list = [1, 2, 3];

foreach ($list as &$value) {
    // ...
}
// forgot unset($value)

foreach ($list as $value) {
    // the second loop writes into the last item!
}

print_r($list);

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Array
(
    [0] => 1
    [1] => 2
    [2] => 2
)

The last item should be 3, but it became 2. The first loop left $value pointing at the last item. The second loop then wrote each value into it. Fix it with unset($value); after the first loop.

5. Doing the same work in every round

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// Slow: count() is called again and again
for ($i = 0; $i < count($bigArray); $i++) { /* ... */ }

// Better: compute it once
$total = count($bigArray);
for ($i = 0; $i < $total; $i++) { /* ... */ }

For arrays, PHP makes count() fast, but the idea matters for anything slower, such as a database query. Never run a database query inside a loop if one query can fetch everything. See Avoiding Repeated Work.

6. Using the loop variable afterwards

After a for or foreach, the variable keeps its last value. That can confuse readers and cause bugs.

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<?php
foreach ([10, 20, 30] as $n) {}

echo $n;

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30