Hi, i have similar problem i have post a topic but after edit it disappeared.
FILE: /home/*******/public_html/wp-content/plugins/wpdm-pdf-stamper/libs/fpdf1.7/fpdf.php
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FOUND 1 ERROR AND 1 WARNING AFFECTING 2 LINES
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76 | WARNING | Use of deprecated PHP4 style class constructor is not supported since PHP 7.
1055 | ERROR | Function set_magic_quotes_runtime() is deprecated since PHP 5.3 and removed since PHP 7.0
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FILE: /home/*******/public_html/wp-content/plugins/wpdm-pdf-stamper/libs/fpdi1.5.2/filters/FilterLZW.php
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FOUND 1 ERROR AFFECTING 1 LINE
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156 | ERROR | Bitwise shifts by negative number will throw an ArithmeticError in PHP 7.0
Not sure maybe it will work and this is a false positive, let me know if is safe to update PHP version to 7 thank you.