Checa se um valor existe em uma string

readme.md

String (needle) upper and lower case letters are different.

function.php

<?php

function in_string( $needle, $haystack ){
	if( !isset( $needle ) || isset( $needle ) && !is_string( $needle ) ){
		$error['needle'] = 'Param $needle invalid';
	}

	if( !isset( $haystack ) || isset( $haystack ) && !is_string( $haystack ) ){
		$error['$haystack'] = 'Param $needle invalid';
	}

	if( isset( $error ) ){
		trigger_error( $error, E_USER_ERROR );
	}

	$needle = str_replace( array(
		'$', '/', "'", '"', '^', '*', '+', '-', '{', '}', '(', ')', 
		',', '.', '?', '!', '@', '#', '&', '[', ']', ';', '<', '>',
		'`', '´', '~', '%', '-', '=', '|'
	), array(
		'\$', '\/', "'", '"', '\^', '\*', '\+', '\-', '\{', '\}', '\(', '\)',
		'\,', '\.', '\?', '\!', '\@', '\#', '\&', '\[', '\]', '\;', '\<', '\>',
		'\`', '\´', '\~', '\%', '\-', '\=', '\|'
	), strip_tags( $needle ) );

	$in_string = array_filter( preg_split( "/$needle/", $haystack ) );

	if( empty( $in_string ) ){
		return true;
	}

	return false;
}

$haystack = 'ababcd';
$needle   = 'ababcd';

var_dump( in_string( $needle, $haystack ) );