header.php Wordpress Child Theme add a line of code

I want to create a header.php file in my child theme and add a line of code.  I know I can simple copy and paste the enitre header.php file and then add lines of code but that seems wrong to me.  Should I be writing hooks instead.  Here is the line of code and the line of code I want to add.

<a href="<?php echo esc_url( home_url( '/' ) ); ?>">
<img src="<?php echo esc_attr( $logo ); ?>" alt="<?php echo esc_attr( get_bloginfo( 'name' ) ); ?>" id="logo" />

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<p id="slogan"><?php bloginfo( 'description' ); ?></p>

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All I want to do is insert the slogan line of code above the first line of code.  Can I write a hook to do this?  Please send the code as I don't know how to write hooks but want to learn.
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Julian HansenCommented:
Not sure how your code is hanging together but you can do something like this.

In your header.php file you do the following - add this to where the slogan should appear.

<?php echo empty($slogan) ? '' : $slogan;?>

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Now in your child theme before you include header.php
$slogan = <<< HTML
<p id="slogan"><?php bloginfo( 'description' ); ?></p>


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In other words you make provision for a $slogan in your header but it only output's it if it has been defined. You then define (set) it before you include the header. In pages that don't require a slogan it won't be defined and therefore nothing will be output in the slogan position.

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livewirewebsolutionsAuthor Commented:
yes, but what if the header.php file is updated in a theme update?  Won't the first code disappear?
Julian HansenCommented:
Not necessarily, word press child themes are by default meant to provide the facility to create theme changes that won't be overwritten.
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