String Concat In Loop

ID

php.string_concat_in_loop

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Php

Tags

efficiency, loop

Description

Reports repeated string concatenation that accumulates into the same variable inside a loop, either with the .= operator ($out .= $part;) or the plain $out = $out . $part; form.

Rationale

PHP strings are immutable, so every concatenation allocates a new string and copies the whole accumulated content into it. Doing this once per iteration makes the cost grow with the square of the number of iterations. Collecting the pieces in an array and joining them once with implode() after the loop runs in linear time and is usually clearer as well.

<?php
$out = '';
foreach ($parts as $part) {
    $out .= $part;          // FLAW — quadratic concatenation in the loop
}

$pieces = [];
foreach ($parts as $part) {
    $pieces[] = $part;      // OK — collect the parts
}
$out = implode('', $pieces); // OK — join once after the loop

Remediation

Append the pieces to an array inside the loop and call implode() once after it. Augmented arithmetic such as $total += $n is unaffected — only string concatenation (.= or $x = $x . …​) is reported.

References