break line inside string

First, be aware of the difference between characters and bytes.

The function strlen() returns amount in bytes.

CAso want to count the number of characters, use the function mb_strlen()

$str = "Vou para Maranhão";
echo strlen($str); //exibe 18
echo mb_strlen($str); //exibe 17

Let's go to the main point?

count the number of characters by ignoring line breaks:

$str = "Vou para 
Maranhão";

$l = mb_strlen( str_replace("
",'',$str) );
echo PHP_EOL . $l; //exibe 17

The idea is simple. Just remove unwanted characters before counting.

By playing a bit more, we can create a function:

$str = "Vou para 
Maranhão";

/**
No segundo parâmetro, indique os caracteres que dseja ignorar. O argumento recebe 'string' ou 'array'
*/
function mb_strlen2( $str, $ignore = null )
{
    return mb_strlen( str_replace($ignore,'',$str) );
}

/*
 Ignora quebras de linha
*/
echo PHP_EOL . '<br />' . mb_strlen2( $str, PHP_EOL );

/*
 Ignora quebras de linha e espaçamentos
*/
echo PHP_EOL . '<br />' . mb_strlen2( $str, [ PHP_EOL, ' ' ] );

Try

    <?php 

    $str = "Vou para Manaus";               
    echo(strlen($str)); //exibe 15

    $str = "Vou para \n Manaus";                
    echo(strlen($str)); //exibe 18


?> 

Try this

$str = "Vou para "\n" Manaus";                
echo(strlen($str)); //exibe 18