Creating comma-separated '?' placeholders for SQL query parameters

Xiaoy312's suggestion is one of the common methods used to generate comma-separated placeholder strings. It generates an array containing ? valued elements, then joins them into a string using commas as glue.

Another common method is:

rtrim(str_repeat('?,',sizeof($arr)),',');

or the equivalent:

substr(str_repeat('?,',sizeof($arr)),0,-1);

My above methods directly generate a string (no temporary array) then merely trims the last character (comma) off the end.

All three mentioned methods will use 3 functions and no conditionals. All, in my opinion, are equally elegant and in all reasonable cases the performance will seem equal to the end user.

In my few simple tests on 3v4l.org, I found that the rtrim(str_repeat(sizeof())) method is slightly faster than Xiaoy's implode(array_fill(),sizeof()) method and my substr(str_repeat(sizeof()) method.

I mean you could go with: implode(',',array_map(function(){return '?';},$arr)); but it seems less elegant and is less efficient.

rtrim(array_reduce($arr,function($carry){return $carry.'?,';}),','); is surprisingly efficient but like array_map() it is less compact/elegant than other fill/repeat methods.

I don't have any formal benchmark results to post. Like all speed tests, the only relevant information for your project(s) is performance in your exact environment. Always do your own benchmarks, but be careful not to waste time micro-optimizing.

As an aside, I would like to suggest some micro-optimizations that I use as a matter habit...

When you are writing for loops, always cache the count() by setting it to a variable in the first expression. This will ensure that you aren't calling count() on every iteration.

for($i = 0, $count = count($arr); $i < $count; $i++)

When the result of pre-incrementing and post-incrementing is the same, I use pre-incrementing as the default because I read that it is ever-so-slightly faster.

for($i = 0, $count = count($arr); $i < $count; ++$i)

When you are assigning array keys starting from 0, you can let php index the array for you.

$question[] = '?';

When you want to loop through all elements in an array, you can use foreach() to avoid counting the array size as well as declaring and incrementing a "counter".

foreach($arr as $v){
    $question[]='?';
}

Given your starting array, which appears to be a field-value map, I find it odd that you want to go with question-mark-style query parameters instead of named placeholders. You might find named placeholders more useful should you ever need to do some debugging around your queries.

This can also simplify your SQL preparation. For example, one might typically build field and/or parameter strings directly from an input array like you have shown.

You can then also work directly with such an array when executing the prepared statement.

For example in PDO:

$paramArray = [];
foreach($inputArray as $key => $value) {
    $paramArray[':' . $key] = $value;
}
$fieldString = '`' . implode('`,`' array_keys($inputArray)) . '`';
$paramString = implode(',', array_keys($paramArray));
$sql = "
    INSERT INTO
    table ({$fieldString})
    VALUES ({$paramString})
";
$stmt = $pdo->prepare($sql);
$stmt->execute($paramArray);