PHP 7 compatible?

I though you might like to know I got this error message when checking php7 compatibility:
FILE: /var/www/vhosts/oneill-injurysolicitors.co.uk/httpdocs/wp-content/plugins/collapsing-categories/collapscatlist.php
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FOUND 1 ERROR AFFECTING 1 LINE
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542 | ERROR | Extension ‘mysql_’ is deprecated since PHP 5.5 and removed since PHP 7.0; Use mysqli instead
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