Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.php

<?php
$arr = [1, 2, 4, 4, 4, 7, 9];
$target = 4;
$lo = 0;
$hi = count($arr) - 1;
$result = -1;
while ($lo <= $hi) {
	$mid = intdiv($lo + $hi, 2);
	if ($arr[$mid] == $target) {
		$result = $mid;
		$hi = $mid - 1;
	} elseif ($arr[$mid] < $target) {
		$lo = $mid + 1;
	} else {
		$hi = $mid - 1;
	}
}
echo $result . "\n";

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This PHP DSA version keeps the same data and final output as every other DSA book in this wave.