Show that strpos() on a chunk using substr() is more efficient than on the whole for large strings.

strpos-chunk.php

function find( ?string $string )
{
	static $found = 0;

	if( null === $string ) return $found;

	$found += 0 === strpos( $string, BOM_UTF8 );
	$found += 0 === strpos( $string, BOM_UTF16_BE );
	$found += 0 === strpos( $string, BOM_UTF16_LE );
	$found += 0 === strpos( $string, BOM_UTF32_BE );
	$found += 0 === strpos( $string, BOM_UTF32_LE );
}

function random_string( int $length = 32, string $pool = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz', string $encoding = null ) : string
{

	if( $length === 0 )
		return '';

	if( $length < 0 )
		throw new DomainException( 'Length must be >= 0.' );

	if( $pool === '' )
		throw new DomainException( 'Pool cannot be an empty string.' );

	$encoding = (array) $encoding;
	$maxKey   = mb_strlen( $pool, ...$encoding );

	if( $maxKey === false )
		throw new DomainException( 'Invalid encoding specified.' );

	for( --$maxKey, $random = ''; $length-- ; )
		$random .= mb_substr( $pool, random_int( 0, $maxKey ), 1, ...$encoding );

	return $random;
}

const BOM_UTF8     = "\xEF\xBB\xBF";
const BOM_UTF16_BE = "\xFE\xFF";
const BOM_UTF16_LE = "\xFF\xFE";
const BOM_UTF32_BE = "\x00\x00\xFE\xFF";
const BOM_UTF32_LE = "\xFF\xFE\x00\x00";

$bomLess = random_string( 1024 * 1024  );
$tests   = 1;

$full = -hrtime( true );
for( $i = 0; $i < $tests; ++$i )
	find( $bomLess );
$full += hrtime( true );

$chunk = -hrtime( true );
for( $i = 0; $i < $tests; ++$i )
	find( substr( $bomLess, 0, 4 ) );
$chunk += hrtime( true );

var_dump(
	'Found: ' . find( null ),
	"Finding on full took:  $full",
	"Finding on chunk took: $chunk"
);