proj4php error with the conversion from epsg 3857 to 4326

Question

$GLOBALS['proj4']->addDef("EPSG:3857","+title=WGS 84 / Pseudo-Mercator +proj=merc +a=6378137 +b=6378137          +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs");
//PROIEZIONE ORIGINE:
$GLOBALS['projGB'] = new proj4php\Proj('EPSG:3857', $GLOBALS['proj4']);
//PROIEZIONE DESTINAZIONE:
$GLOBALS['projWGS84'] = new proj4php\Proj('EPSG:4326', $GLOBALS['proj4']);

error: failed to find projection file for: (string) merc *

I've tried to make the conversion from EPSG:3857 TO EPSG:4326. It's the first time i use this library and i don't know if i make some mistakes, can you please help me? I have attached also the type of error for this conversion.