Unable to run 64-bit PHP 7.2 on 64-bit Linux server (Drupal installation)

Status Report gives me the following warning :
LIMITED DATE RANGE : Your PHP installation has a limited date range.
You are running on a system where PHP is compiled or limited to using 32-bit integers. This will limit the range of dates and timestamps to the years 1901-2038... 

A few searches revealed that this warning is generated if the value of the PHP predefined constant PHP_INT_SIZE <= 4.  I checked, and this was indeed so (the value was 4, i.e. 32 bits). I was also running on PHP 5 at the time.

I have since purged the old PHP, installed the latest – PHP 7.2 – but I still get this warning and the value of PHP_INT_SIZE is still 4 (32 bits) while I have a Debian Stretch 64-bit server. The lscpu command output (posted at the end confirms this).

I know this is not strictly a Drupal question, but I am sure many of you may be able to suggest how to proceed to get PHP 7.2 working in 64-bit mode (which it should by default). Also, perhaps the presence of Drupal which was installed while PHP 5.5 was running may be causing the newer PHP 7 to install itself in 32-bit mode?

Here’s the lscpu output :

Architecture:          i686
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    2
Socket(s):             1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 15
Model name:            Intel(R) Xeon(R) CPU            3050  @ 2.13GHz
Stepping:              2
CPU MHz:               2133.299
BogoMIPS:              4266.59
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              2048K
Flags:                 fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon pebs bts aperfmperf eagerfpu pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm lahf_lm tpr_shadow dtherm
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