test_array_group_c.php

<?php

$start = microtime( true );

$book_sections = [ '1.0', '1.1', '1.2', '2.0', '2.1', '3.0', '3.1' ];
$groups = array_group_pair( $book_sections, function( $p1, $p2 ) {
	return $p1[0] === $p2[0];
} );

$end = microtime( true );

printf( "%.10f\n", $end - $start );

test_array_group_php.php

<?php
$start = microtime( true );
$book_sections = [ '1.0', '1.1', '1.2', '2.0', '2.1', '3.0', '3.1' ];

function custom_array_group(array $arr1, callable $compare): array {
    $groups = [];
    $group = [];
    $prev = NULL;
    foreach ($arr1 as $value) {
        if ($group && !$compare($prev, $value)) {
            $groups[] = $group;
            $group = [];
        }
        $group[] = $value;
        $prev = $value;
    }
    if ($group) {
        $groups[] = $group;
    }
    return $groups;
}

$groups = custom_array_group( $book_sections, function( $p1, $p2 ) {
	return $p1[0] === $p2[0];
} );

$end = microtime( true );

printf( "%.10f\n", $end - $start );

$ for i in {1..100}; do sapi/cli/php test_array_group_php.php; done | awk '{sum += $1} END {print sum/100}'
3.22533e-05
$ for i in {1..100}; do sapi/cli/php test_array_group_c.php; done | awk '{sum += $1} END {print sum/100}'
2.44975e-05

An improvement of (3.22533 - 2.44975)/3.22533 = 24%